Theoretical and experimental insights into the surface chemistry of semiconductor quantum dots - Margraf, Johannes; Ruland Palaia, Andres; Sgobba, Vito; Guldi, Dirk Michael; Clark, Timothy (Langmuir, 2013)
We present a series of non-stoichiometric cadmium sulfide quantum-dot (QD) models. Using density functional theory (DFT) and semi-empirical molecular orbital (MO) calculations, we explore the ligand binding and exchange chemistry of these models. Their surface morphology allows for these processes to be rationalized on the atomic scale. This is corroborated by ultraviolet-visible (UV-vis), infrared (IR), and inductively coupled plasma-optical emission spectroscopy (ICP-OES). © 2013 American Chemical Society.

The path to ubiquitous organic electronics hinges on its stability - Brabec, Christoph; Egelhaaf, Hans-Joachim; Salvador, Michael (Journal of Materials Research, 2018)

The Physics of Small Molecule Acceptors for Efficient and Stable Bulk Heterojunction Solar Cells - Gasparini, Nicola; Wadsworth, Andrew; Moser, Maximilian; Baran, Derya; Mcculloch, Iain; Brabec, Christoph (Advanced Energy Materials, 2018)
Organic bulk heterojunction solar cells based on small molecule acceptors have recently seen a rapid rise in the power conversion efficiency with values exceeding 13%. This impressive achievement has been obtained by simultaneous reduction of voltage and charge recombination losses within this class of materials as compared to fullerene-based solar cells. In this contribution, the authors review the current understanding of the relevant photophysical processes in highly efficient nonfullerene acceptor (NFA) small molecules. Charge generation, recombination, and charge transport is discussed in comparison to fullerene-based composites. Finally, the authors review the superior light and thermal stability of nonfullerene small molecule acceptor based solar cells, and highlight the importance of NFA-based composites that enable devices without early performance loss, thus resembling so-called burn-in free devices.
Keywords: charge recombination; nonfullerene acceptors; organic solar cells; photophysics; stability

The potential of Rutherford Backscattering Spectrometry for composition analysis of colloidal nanocrystals - Primetzhofer, Daniel; Sytnyk, M.; Wagner, P. J.; Bauer, P.; Heiß, Wolfgang (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2014)
We investigate the potential of elastic scattering of energetic ions for compositional analysis of magnetic colloidal nanocrystals. Thin layers of nanocrystals deposited by spin coating on Si-wafers are investigated by two different set-ups for Rutherford Backscattering Spectrometry (RBS), employing different projectile ions (He-4, C-12) and primary energies (600 keV-8 MeV). The advantages and disadvantages of the different approaches are discussed in terms of obtainable mass resolution, necessary primary particle fluence and deposited energy. It is shown that different isotopes of transition metals can be resolved by employing 8 MeV C-12(3+) primary ions. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: RBS; Elastic scattering; Nanocrystal; Cation-exchange; C

The processing chain of the wide bandgap semiconductor SiC – How small steps enabled a mature technology - Wellmann, Peter; Steiner, Johannes; Strüber, Sven; Arzig, Matthias; Salamon, Michael; Uhlmann, Norman; Nguyen, Binh Duong; Sandfeld, Stefan (Diamond and Related Materials, 2023)
This work paper was presented as a keynote lecture at the international conference on diamond and related materials in Lisbon (Portugal) in the year 2022. This paper summarizes in the first part the processing chain of the semiconductor material SiC from the raw material to epitaxially-ready wafers as they are used for electronic device manufacturing. In the second part a current research study, the reduction of the basal plane dislocation density in SiC crystal growth is presented. Among other defects, basal plane dislocations belong to the more severe structural defects in SiC with respect to degradation during electronic device operation. In the third part the applicability of X-ray topography to reveal dislocations and other structural defects in SiC is outlined in a review style.
CRIS-Team Scopus Importer:2023-04-14
Keywords: Bulk crystal growth; Dislocations; Processing chain; Silicon carbide

The Relevance of Solubility and Miscibility for the Performance of Organic Solar Cells - Langner, Stefan; Perea Ospina, Jose Dario; Zhang, Chaohong; Li, Ning; Brabec, Christoph (CRC Press, 2019)
This chapter introduces the approach to ab initio calculate the values based on computational methods, and a Figure of Merit to predict the microstructure stability of bulk heterojunction (BHJ) composites. It focuses on solute-liquid miscibility and solute-solute miscibility and explains the degradation mechanisms related to microstructure. The chapter provides a detailed insight into the theoretical, computational and experimental principles of mixing of organic semiconductors. The microstructure of a BHJ film is determined by the competition between thermodynamics and kinetics during the drying process. The chapter describes the M. L. Hansen solubility parameters (HSPs) can be visualized in a three dimensional coordinates system with the solubility parameters as axes. The mixing compatibility of a polymer and a fullerene is graphically represented by the values of thermodynamic variables when the equilibrium is established among the phases of a system.
CRIS-Team Scopus Importer:2023-03-07

The Relevance of Solubility and Miscibility for the Performance of Organic Solar Cells - Langner, Stefan; Perea Ospina, Jose Dario; Zhang, Chaohong; Li, Ning; Brabec, Christoph (Conjugated Polymers. Properties, Processing, and Applications, 2019)
This chapter introduces the approach to ab initio calculate the values based on computational methods, and a Figure of Merit to predict the microstructure stability of bulk heterojunction (BHJ) composites. It focuses on solute–liquid miscibility and solute–solute miscibility and explains the degradation mechanisms related to microstructure. The chapter provides a detailed insight into the theoretical, computational and experimental principles of mixing of organic semiconductors. The microstructure of a BHJ film is determined by the competition between thermodynamics and kinetics during the drying process. The chapter describes the M. L. Hansen solubility parameters (HSPs) can be visualized in a three dimensional coordinates system with the solubility parameters as axes. The mixing compatibility of a polymer and a fullerene is graphically represented by the values of thermodynamic variables when the equilibrium is established among the phases of a syst
CRIS-Team WoS Importer:2021-10-18

Thermal-Driven Phase Separation of Double-Cable Polymers Enables Efficient Single-Component Organic Solar Cells - Feng, Guitao; Li, Junyu; He, Yakun; Zheng, Wenyu; Wang, Jing; Li, Cheng; Tang, Zheng; Osvet, Andres; Li, Ning; Brabec, Christoph; Yi, Yuanping; Yan, He; et al. (Joule, 2019)
Conventional organic solar cells (OSCs) need two components that function as donor and acceptor, respectively. Although there has been wishful thinking about constructing OSCs based on a single component, it is generally believed to be highly challenging to achieve efficient single-component OSCs (SCOSCs). In this work, we design a new double-cable conjugated polymer containing a strongly crystalline backbone as donor and aromatic side units as acceptor. With a high annealing temperature (230°C), both the backbones and perylene bisimide side units could self-organize into ordered nanostructures. This enables efficient charge transport and low charge recombination, resulting in a record efficiency of 6.3% in SCOSCs. The cells also exhibit excellent stability, with >90% efficiency retention over 300 h of continuous one-sun illumination. These results suggest that the concept of SCOSCs is highly promising, especially to overcome the limitations of conventional OSCs toward industrial application.
CRIS-Team Scopus Importer:2019-07-19
Keywords: conjugated polymers; double cable; nanophase separation; single-component organic solar cells; thermal driven

Thermal Expansion Coefficients of 6H Silicon Carbide - Stockmeier, Matthias; Sakwe, Aloysius; Hens, Philip; Wellmann, Peter; Hock, Rainer; Magerl, Andreas (Materials Science Forum, 2009)
The thermal expansion of 6H Silicon Carbide with different dopant concentrations of aluminum and nitrogen was determined by lattice parameter measurements at temperatures from 300 K to 1575 K. All samples have a volume of at least 6 x 6 x 6 mm(3) to ensure that bulk properties are measured. The measurements were performed with a triple axis diffractometer with high energy x-rays with a photon energy of 60 keV. The values for the thermal expansion coefficients along the a- and c-direction, alpha(11) and alpha(33), are in the range of 3.10(-6) K(-1) for 300 K and 6.10(-6) K(-1) for 1550 K. At high temperatures the coefficients for aluminum doped samples are approximately 0.5.10(-6) K(-1) lower than for the nitrogen doped crystal. alpha(11) and alpha(33) appear to be isotropi
Keywords: Bulk Characterization; High Energy X-Ray Diffraction; Thermal Expansion

Thermally Stimulated Luminescence in 6H Fluorescent SiC - Wei, Yi; Künecke, Ulrike; Wellmann, Peter; Ou, Haiyan (International Conference on Silicon Carbide and Related Materials, 2017)
Fluorescent silicon carbide (f-SiC) is the emerging material for white light-emitting diodes (LED) thanks to its high conversion efficiency from near-ultraviolet (NUV) to visible light [1-2]. However, the procedures of the donor-acceptor pair (DAP) recombination, which plays the key role for the wavelength conversion of the f-SiC material and determines the internal quantum efficiency (IQE), have not been precisely interpreted yet. The occupancy conditions of the electrons and the holes on the different sites [3] of the donor levels and the acceptor levels respectively vary with different f-SiC samples (e.g., 2 types of cubic sites and 1 type of hexagonal sites of the donor levels for 6H SiC), and the dominance of the DAP recombination process is site-dependent [4], hence the tuning of the dominant occupancies of the electrons and the holes to enhance the DAP procedures is an important issue during the crystal growth and is able to essentially improve the IQE of the 6H f-SiC material. In this paper, we demonstrate the characterization of the dominant occupancies of the electrons on the donor levels of the two different n-type 6H f-SiC samples by using thermally stimulated luminescence (TSL). TSL has been applied for the experimental characterization, since TSL has no requirement of contact preparation for samples which is quite time-consuming, making the advantage of the TSL method beyond the conventional defect levels characterization methods like deep level transient spectroscopy [5]. We further theoretically studied the measured TSL results based on the monomolecular and bimolecular thermal activation energy model developed by Halperin A. et al. [6] with our own modification according to the special case for the f-SiC material. The modelled TSL curves fitted well with the measured TSL curves regarding to both the line shape and the actual TSL intensities<br
Keywords: Fluorescent SiC; 6H SiC; TSL

The role of chemical structure in indacenodithienothiophene-alt-benzothiadiazole copolymers for high performance organic solar cells with improved photo-stability through minimization of burn-in loss - Chochos, Christos L.; Leclerc, Nicolas; Gasparini, Nicola; Zimmerman, Nicolas; Tatsi, Elisavet; Katsouras, Athanasios; Moschovas, Dimitrios; Serpetzoglou, Efthymis; Konidakis, Ioannis; Fall, Sadiara; Leveque, Patrick; Heiser, Thomas; et al. (Journal of Materials Chemistry A, 2017)
It is of upmost importance to gain an in-depth understanding of the role of the polymer chemical structure in the performance of the corresponding organic solar cell (OSC) and its degradation behavior, which is currently insufficiently explored. Achieving these correlations will set new design rules towards further optimization of polymer chemical structures for OSCs exhibiting high performances and long stability. In this study, our efforts have been focused on identifying how the nature of aryl substituents and the number of fluorine atoms anchored in the backbone of indacenodithieno[3,2-b]thiophene (IDTT) based polymers influence their optoelectronic properties, the OSC performances and their degradation mechanisms. The most important outcome of this study is the demonstration that standard initial burn-in loss is primary attributed to microstructure instabilities. Furthermore, the initial burn-in loss could be substantially reduced through the rational design of the polymeric semiconductor's chemical structure, leading to improved lifetimes and low (20%) initial power conversion efficiency loss. In particular, we identify the beneficial effect of the presence of the two fluorine atoms on the benzo[c][1,2,5]thiadiazole (BTD), as compared to the non-fluorinated and mono-fluorinated analogues, in decreasing the burn-in by reducing the microstructure instabilities regardless of the aryl substituent that is present in the polymer backbone. © The Royal Society of Chemistry.
Keywords: Engineering controlled terms: Chemical stabilityDegradationFluorineImage enhancementMicrostructureOrganic solar cellsPolymer solar cellsPolymersSolar cellsStructure (composition)Substitution reactions Compendex keywords Beneficial effectsBenzothiadiazolesDegradation behaviorDegradation mechanismIn-depth understandingOptoelectronic propertiesPolymeric semiconductorsPower conversion efficiencies Engineering main heading: Structural design

The role of connectivity in significant bandgap narrowing for fused-pyrene based non-fullerene acceptors toward high-efficiency organic solar cells - Liu, Shungang; Su, Wenyan; Zou, Xianshao; Du, Xiaoyan; Cao, Jiamin; Wang, Nong; Shen, Xingxing; Geng, Xinjian; Tang, Zilong; Yartsev, Arkady; Zhang, Maojie; Gruber, Wolfgang; et al. (Journal of Materials Chemistry A, 2020)

The role of defects in fluorescent silicon carbide layers grown by sublimation epitaxy - Schimmel, Saskia; Kaiser, Michl; Jokubavicius, Valdas; Ou, Yiyu; Hens, Philip; Linnarsson, Margareta K.; Sun, J.; Liljedahl, Rickard; Ou, Haiyan; Syväjärvi, Mikael; Wellmann, Peter (IOP Conference Series: Materials Science and Engineering, 2014)
Donor-acceptor co-doped SiC is a promising light converter for novel monolithic all-semiconductor white LEDs due to its broad-band donor-acceptor pair luminescence and potentially high internal quantum efficiency. Besides sufficiently high doping concentrations in an appropriate ratio yielding short radiative lifetimes, long nonradiative lifetimes are crucial for efficient light conversion. The impact of different types of defects is studied by characterizing fluorescent silicon carbide layers with regard to photoluminescence intensity, homogeneity and efficiency taking into account dislocation density and distribution. Different doping concentrations and variations in gas phase composition and pressure are investigated.
Keywords: SiC; fluorescence; sublimation epitaxy

The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets - Classen, Andrej; Chochos, Christos L.; Lüer, Larry; Gregoriou, Vasilis G.; Wortmann, Jonas; Osvet, Andres; Forberich, Karen; McCulloch, Iain; Heumüller, Thomas; Brabec, Christoph (Nature Energy, 2020)
Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells.
CRIS-Team Scopus Importer:2020-09-11

The role of innovation for economy and sustainability of photovoltaic modules - Peters, Ian Marius; Hauch, Jens; Brabec, Christoph (iScience, 2022)
The role of innovation for the success of photovoltaics cannot be overstated. Photovoltaics have enjoyed the most substantial price learning of any energy technology. Innovation affects photovoltaic performance in more ways, though. Here, we explore the role of innovation for economics and greenhouse gas savings of photovoltaic modules using replacement scenarios. We find that the greenhouse gas displacement potential of photovoltaic modules has improved substantially over the last 20 years—4-fold for the presented example. We show that the economically ideal time for repowering is after around 20 years, but that repowering may reduce greenhouse gas savings. Expanding photovoltaic installations is generally preferable, economically and sustainably, to repowering. We argue that i) we should maximize the greenhouse gas saving potential of each module, which requires a global strategy, ii) tandem solar cells should aim for stability, and iii) efforts to continue and accelerate innovation in photovoltaic technology are needed.
CRIS-Team Scopus Importer:2022-10-21
Keywords: Energy management; Engineering

The search for new materials and the role of novel processing routes - Wellmann, Peter (Discover Materials, 2021)
Throughout human history, most further developments or new achievements were accompanied by new materials or new processes that enabled the technologic progress. With concrete devices and applications in mind, synthesis and subsequent treatment of materials naturally went along with the progress. The aim of the underlying article is to spot the role of optimization, of discovery, of trial-and-error approaches, of fundamentals and curiosity driven design and development. In a consecutive examination, five missions addressing the challenges facing our world (identified by the European Council) will be cross linked with seven topical areas from materials science defined by the European Materials Research Society. The scope of this examination is to identify approaches and methods to further develop and innovate materials which form the basis of the anticipated solutions.
CRIS-Team Scopus Importer:2024-12-16

The Voltage-Ear – An Anomaly in photovoltaic systems with undersized inverters - Wittmann, Ernst; Buerhop-Lutz, Claudia; Christlein, Vincent; Hauch, Jens; Brabec, Christoph; Peters, Ian Marius (IEEE Photonics Journal, 2024)
This study presents a phenomenon in photovoltaic field installations we call voltage ear. PV installations exhibit the voltage ear under specific conditions where modules generate more power than the inverter can handle. When inverters exceed their power limit it results in inverter clipping, and the solar modules are regulated to operate at a voltage above that at maximum power. This kind of voltage rise leads to a specific pattern in the monitoring data – the voltage ear. In the present study, data from a solar park in France was analyzed. This park featured a high rate of inverter malfunctions, including overheating and fires, resulting in an estimated 5% failure rate and, ultimately, in a complete replacement of inverters. An automated analysis of the monitoring data revealed 32.000 occurrences of excess voltage in 1.65 million data points (2%). Instances of the phenomena were detected and segmented using a K-means sorting algorithm, which yielded a perfect recall value. Analysis of the identified voltage ear conditions showed, that it occurs predominantly during seasons of high irradiance and cool temperatures. Excess voltages of up to 161 V in single strings were observed, increasing the probability for PID and inverter damage. The direct yield loss due to deviation from maximum power point was between 0.02% in January and 0.9% in June. We assume, that the voltage ear is a symptom of system design issues that can result in malfunctioning of inverters and hope that it will be useful as an indicator for identifying design and operation issues.<br
Keywords: Inverters; Ear; Voltage measurement; Monitoring; Voltage control; Temperature measurement; Current measurement; PV field inspection; monitoring data; voltage ear; K-means clustering; inverter failure

Thin-Film Electrostatic Discharge Protection for Highly Segmented OLEDs in Automotive Applications - Bechert, Hermann; Almora, Osbel; Regau, Kilian; Matt, Gebhard; Brabec, Christoph; Wehlus, Thomas (Advanced Materials Technologies, 2019)
Future autonomous driving requires new lighting solutions. Communication in between other cars and pedestrians, which fulfills the requirements of the automotive lighting industry, is needed. A very promising lighting solution for this application are highly segmented organic light-emitting diodes (OLEDs). Unfortunately, small area OLEDs are very sensitive to electrostatic discharge due to the small capacitance of the OLED segments. This study presents a solution for highly segmented OLEDs to fulfill automotive requirements regarding electrostatic discharge (ESD) without cost driving external components but through the improvement of the OLED itself. This solution is designed to be cheap and simple in fabrication, derived from standard photolithography to ensure no impact on existing supply chains and flexible OLED fabrication. After introducing the improved device concept for highly segmented OLEDs and its boundary conditions, a detailed view on the corresponding fabrication steps is given. The capacitive response of the protected OLED segments with respect to OLEDs without the introduced protection layer is characterized via impedance spectroscopy. The results exhibit the functionality of the ESD protection layer and prove with automotive ESD stability requirements complianc
Keywords: OLED; segmentation; ESD protection; impedance spectroscopy; capacitance; automotive rear-light

Thin Film Encapsulation of Organic Solar Cells by Direct Deposition of Polysilazanes from Solution - Channa, Iftikhar Ahmed; Distler, Andreas; Zaiser, Michael; Brabec, Christoph; Egelhaaf, Hans-Joachim (Advanced Energy Materials, 2019)
Organic electronic devices (OEDs), e.g., organic solar cells, degrade quickly in the presence of ambient gases, such as water vapor and oxygen. Thus, in order to extend the lifetime of flexible OEDs, they have to be protected by encapsulation. A solution-based encapsulation method is developed, which allows the direct deposition of the diffusion barrier on top of OEDs, thus avoiding lamination of barrier films. The method is based on the deposition of a perhydropolysilazane (PHPS) ink and its subsequent conversion into a silica layer by deep UV irradiation. The resulting barrier films show water vapor transmission rates (WVTRs) of <10(-2) g m(-2) d(-1) (40 degrees C/85% relative humidity (RH)) and oxygen transmission rates (OTRs) of <10(-2) cm(3) m(-2) d(-1) bar(-1) at ambient conditions. Flexibility of the resulting barrier films is improved by coating a barrier stack of several thin PHPS layers alternating with organic polymer interlayers. These stacks show an increase of WVTR values by less than 10% after 3000 bending cycles. Direct coating of the PHPS films on top of organic solar cells enhances the device lifetime in damp heat conditions from a few hours to beyond 300 h.
CRIS-Team WoS Importer:2019-08-09

Three-dimensional in-situ growth surveillance of bulky SiC crystals - Salamon, Michael; Arzig, Matthias; Wellmann, Peter; Uhlmann, Norman (Proceedings of the DIR 2019, 2019)
The integrated design of modern Computed Tomography systems, optimized to achieve the required environmental conditions for a scan, such as stable Temperature or low vibration has helped to widespread the CT method along non-destructive testing and coordinate measurement applications. For in-situ analysis of processes this integrated design is often limiting the application to processes with relatively low peripheral requirements and small footprint. In general, the in-situ process is adapted to the CT using an experimental environment. In cases a productive environment is required the CT has to adapt to the process and its environment. The complex structure of a physical vapor transport (PVT) reactor represents such a case. Inside an evacuated crucible heated inductively to approx. 2400 ° C, the deposition of silicon and carbon atoms takes place on a seed crystal. Over a period of about 100 to 200 hours, crystal of about 20 - 30 mm in height and currently 150 mm in diameter are grown. The growth kinetics depends strongly on the temperature field and further processes characteristics. The geometry of the interior of the crucible and the growth interface of the crystal are relevant parameters that can be observed during the process by means of computed tomography. The publication presents the integration of a CT system into a PVT crystal growing process with a crystal diameter of 75 mm and the challenges related to the absorption characteristics of the bulky crystal and the quality losses resulting from the applied region of interest scanning.<br
Keywords: Computed tomography; in situ analysis; crystal growth; SiC; physical vapour transport

Time-dependent morphology evolution of solution-processed small molecule solar cells during solvent vapor annealing - Min, Jie; Jiao, Xuechen; Ata, Ibrahim; Osvet, Andres; Ameri, Tayebeh; Baeuerle, Peter; Ade, Harald; Brabec, Christoph (Advanced Energy Materials, 2016)
Morphological modification using solvent vapor annealing (SVA) provides a simple and widely used fabrication option for improving the power conversion efficiencies of solution-processed bulk heterojunction (BHJ) small molecule solar cells. Previous reports on SVA have shown that this strategy influences the degree of donor/acceptor phase separation and also improves molecular donor ordering. A blend composed of a dithienopyrrole containing oligothiophene as donor (named UU07) and [6,6]-phenyl-C61-butyric acid methyl ester as acceptor is investigated with respect to SVA treatment to explore the dynamics of the BHJ evolution as a function of annealing time. A systematic study of the time dependence of morphology evolution clarifies the fundamental mechanisms behind SVA and builds the structure-property relation to the related device performance. The following two-stage mechanism is identified: Initially, as SVA time increases, donor crystallinity is improved, along with enhanced domain purity resulting in improved charge transport properties and reduced recombination losses. However, further extending SVA time results in domains that are too large and a few large donor crystallites, depleting donor component in the mixed domain. Moreover, the larger domain microstructure suffers from enhanced recombination and overall lower bulk mobility. This not only reveals the importance of precisely controlling SVA time on gaining morphological control, but also provides a path toward rational optimization of device performance. Time-dependent morphology evolution of solution-processed small molecule solar cells during solvent vapor annealing is systematically and methodically investigated. This not only reveals the importance of precisely controlling SVA time on gaining morphological control, but also provides a path toward the rational optimization of device performance.
Keywords: bulk mobility; device performance; morphology evolution; solar cells; solvent vapor annealing

Time-Resolved Analysis of Dielectric Mirrors for Vapor Sensing - Gao, Shuai; Tang, Xiaofeng; Langner, Stefan; Osvet, Andres; Harreiß, Christina; Barr, Maissa; Spiecker, Erdmann; Bachmann, Julien; Brabec, Christoph; Forberich, Karen (ACS Applied Materials and Interfaces, 2018)
Dielectric mirrors based on bilayers of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) and poly(vinyl alcohol) (PVA)-zirconium dioxide (ZrO2) nanocomposites are fabricated for vapor sensing. When exposed to specific solvent vapor, the layers of dielectric mirrors can gradually swell and cause a red-shift of the reflection band. Because PVA solely responds to water and SEBS is sensitive to several different types of organic solvents, the mirrors can respond to a large variety of solvents. The dual-functional hydrophilic ZrO2 nanoparticles are introduced to not only enlarge the refractive index contrast but also increase the permeability. Time-resolved measurements show that mirrors with nanoparticles have a significantly faster response than those without nanoparticles. Moreover, the dependence on relative humidity is studied for representative solvents, and several types of solvents are selected to show the dependence on the solvent-polymer interaction parameters at typical relative humidity, which allows one to predict the responsivity and selectivity of the sensors. © 2018 American Chemical Societ
Keywords: dielectric mirrors; distributed Bragg reflectors; vapor sensing; fully printed; time-resolved analysis

Time-resolved imaging of Œrsted field induced magnetization dynamics in cylindrical magnetic nanowires - Schöbitz, M.; Finizio, S.; De Riz, A.; Hurst, J.; Thirion, C.; Gusakova, D.; Toussaint, J. C.; Bachmann, Jonas; Raabe, J.; Fruchart, O. (Applied Physics Letters, 2021)
Recent studies in three-dimensional spintronics propose that the Œrsted field plays a significant role in cylindrical nanowires. However, there is no direct report on its impact on magnetic textures. Here, we use time-resolved scanning transmission x-ray microscopy to image the dynamic response of magnetization in cylindrical Co30Ni70 nanowires subjected to nanosecond Œrsted field pulses. We observe the tilting of longitudinally magnetized domains toward the azimuthal Œrsted field direction and create a robust model to reproduce the differential magnetic contrasts and extract the angle of tilt. Furthermore, we report the compression and expansion, or breathing, of a Bloch-point domain wall that occurs when weak pulses with opposite signs are applied. We expect that this work lays the foundation for and provides an incentive to further studying complex and fascinating magnetization dynamics in nanowires, especially the predicted ultra-fast domain wall motion and associated spin wave emissions.
CRIS-Team Scopus Importer:2021-05-14

TiO2-WO3 Composite Nanotubes by Alloy Anodization: Growth and Enhanced Electrochromic Properties - Nah, Yoon-Chae; Ghicov, Andrei; Kim, Dam-Hee; Berger, Stefan; Schmuki, Patrik (Journal of the American Chemical Society, 2008)
The present work demonstrates that uniform and highly ordered arrays of TiO_2-WO_3 nanotubes can be grown by anodization of Ti alloys in an ethylene glycol/fluoride based electrolyte under selected electrochemical conditions. These aligned mixed oxide nanotube structures are highly suitable for enhanced electrochromic reactions; in particular we show that already small amounts of WO_3 (such as 0.2 at%) present in the tube oxide drastically improve the electrochromic properties (contrast, onset potential, cycling stability) of nanotube layer based devices.
EAM Import::2019-03-13

TiO2 nanotubes and their application in dye-sensitized solar cells - Roy, Poulomi; Kim, Dam-Hee; Lee, Kiyoung; Spiecker, Erdmann; Schmuki, Patrik (Nanoscale, 2010)
The present article reviews the current status of using TiO2 nanotubes in Gratzel-type, dye-sensitized solar cells and extends the overview with the latest results and findings. Critical factors in tube geometry (length, diameter, top morphology), crystal structure (amorphous, anatase, rutile) as well as factors affecting dye loading or electron mobility are addressed. The highest solar cell efficiencies today for pure nanotube systems reach approximately 4% while for some mixed systems, around 7% has been reported. For both Systems significant room for enhancement is anticipated and some key points and strategies for improvement are outlined.

TIPS Pentacene as a Beneficial Interlayer for Organic Photodetectors in Imaging Applications - Montenegro Benavides, Cindy; Biele, Markus; Schmidt, O.; Brabec, Christoph; Tedde, S. F. (IEEE Transactions on Electron Devices, 2018)
The small molecule 6,13-bis (triisopropylsilylethynyl) (TIPS) pentacene has been used as an interlayer for the reduction of dark currents in organic photodetectors (OPDs) based on a bulk heterojunction of poly-3-hexyl-thiophene(P3HT) and [6, 6]-phenyl C61 butyric acid methyl ester. We report OPDs with dark currents of ∼0.9 nA/cm2 at −5 V and external quantum efficiencies close to 80% at 530 nm. Compared to the reference, P3HT interlayer, TIPS pentacene shows a significant increase of the OPD’s detectivity from 3.29 × 1012 to 1.63 × 1013 Jones. Also, the OPD’s dynamic range is improved reducing the limit of signal detection from 132 to 6.3 nW/cm2 under green light irradiation. Singular hysteresis in the I–V characteristic curve has been observed and impedance spectroscopy measurements were carried out to analyze its origin and generate a replacement electrical model for the device at reverse bias. Finally, OPDs with TIPS pentacene interlayer have been integrated on top of amorphous silicon thin-film transistor backplanes to obtain the first OPD-based image sensor using this interlayer.
Keywords: Capacitance; Dark current; Electrodes; Frequency measurement; image sensor; Image sensors; Indium tin oxide; low dark current; organic photodetectors (OPDs); Pentacene; Photodetectors; triisopropylsilylethynyl (TIPS) pentacene interlayer

Towards 15% energy conversion efficiency: A systematic study of the solution-processed organic tandem solar cells based on commercially available materials - Li, Ning; Baran, Derya; Forberich, Karen; Machui, Florian; Ameri, Tayebeh; Turbiez, Mathieu; Carrasco-Orozco, Miguel; Drees, Martin; Facchetti, Antonio; Krebs, Frederik C.; Brabec, Christoph (Energy and Environmental Science, 2013)
Owing to the lack of scalable high performance donor materials, studies on mass-produced organic photovoltaic (OPV) devices lag far behind that on lab-scale devices. In this work, we choose 6 already commercially available conjugated polymers and systematically investigate their potential in organic tandem solar cells. All the devices are processed under environmental conditions using doctor-blading, which is highly compatible with mass-production coating technologies. Power conversion efficiencies (PCE) of 6-7% are obtained for OPV devices based on different active layers. Optical simulations based on experimental data are performed for all realized tandem solar cells. An efficiency potential of ∼10% is estimated for these compounds in combination with phenyl-C-butyric acid methyl ester (PCBM) as an acceptor. In addition, we assume a hypothetical, optimized acceptor to understand the limitation of donors. It is suggested that a PCE of >14% is realistic for tandem solar cells based on these commercially available donor materials. Along with the demonstration of novel intermediate layers we believe that this systematic study provides valuable insight for those attempting to realize the high efficiency potential of tandem architectures. © 2013 The Royal Society of Chemistry.
Keywords: Coating technologies; Environmental conditions; Novel intermediates; Optical simulation; Organic photovoltaic devices; Power conversion efficiencies; Solution-processed; Tandem solar cells Engineering controlled terms: Conversion efficiency Engineering main heading: Solar cells GEOBASE Subject Index: coating; energy efficiency; numerical model; optimization; photovoltaic system

Towards a full understanding of regioisomer effects of indene-C60 bisadduct acceptors in bulk heterojunction polymer solar cells - Cao, Tiantian; Chen, Ning; Liu, Guangxin; Wan, Yingbo; Perea, Jose Dario; Xia, Yijun; Wang, Zhaowei; Song, Bo; Li, Ning; Li, Xiaohong; Zhou, Yi; Brabec, Christoph; et al. (Journal of Materials Chemistry A, 2017)
Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures. © 2017 The Royal Society of Chemistry.
Keywords: Heterojunctions; Mixtures; Open circuit voltage; Photovoltaic effects; Polycyclic aromatic hydrocarbons; Polymer solar cells; Solubility; Bulk heterojunction; Bulk properties; Different structure; Electrochemical studies; Electron acceptor materials; Photovoltaic performance; Polymer solar cell (PSCs); Steric structure; Solar cells

Towards Fully Autonomous Aerial Inspection of Photovoltaic Power Plants - Gallwitz, Florian; Deitsch, Sergiu; Dalsaß, Manuel (Integration of Sustainable Energy Conference (iSEneC), 2016)

Towards large-scale production of solution-processed organic tandem modules based on ternary composites: Design of the intermediate layer, device optimization and laser based module processing - Li, Ning; Kubis, Peter; Forberich, Karen; Ameri, Tayebeh; Krebs, Frederik C.; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2014)
We report on a novel approach including: 1. the design of an efficient intermediate layer, which facilitates the use of most high performance active materials in tandem structure and the compatibility of the tandem concept with large-scale production; 2. the concept of ternary composites based on commercially available materials, which enhances the absorption of poly(3-hexylthiophene) (P3HT) and as a result increase the PCE of the P3HT-based large-scale OPV devices; 3. laser-based module processing, which provides an excellent processing resolution and as a result can bring the power conversion efficiency (PCE) of mass-produced organic photovoltaic (OPV) devices close to the highest PCE values achieved for lab-scale solar cells through a significant increase in the geometrical fill factor. We believe that the combination of the above mentioned concepts provides a clear roadmap to push OPV towards large-scale production and commercial applications. © 2013 Elsevier B.V.
Keywords: Intermediate layer; Laser-based module processing; Optical-simulation; Organic tandem solar cells; Solution processing; Ternary composites

Towards low-cost, environmentally friendly printed chalcopyrite and kesterite solar cells - Azimi, Seyed Hamed; Hou, Yi; Brabec, Christoph (Energy and Environmental Science, 2014)
Solution-processed organic and inorganic semiconductors offer a promising path towards low-cost mass production of solar cells. Among the various material systems, solution processing of multicomponent inorganic semiconductors offers considerable promise due to their excellent electronic properties and superior photo- and thermal stability. This review surveys the recent developments of "all solution-processed" copper-indium (-gallium)-chalcogenide (CuInS, CuInSe and Cu(In, Ga)(Se, S)) chalcopyrites and copper-zinc-tin-chalcogenide (CuZnSnS and CuZnSnSe (CZTS(e))) kesterite solar cells. A brief overview further addresses some of the most critical material aspects and associated loss mechanisms in chalcopyrite and kesterite devices. Today's state-of-the-art performance as well as future challenges to achieve low-cost and environmentally friendly production is discussed. This journal is © the Partner Organisations 2014.
Keywords: Engineering controlled terms: Chalcogenides; Copper compounds; Costs; Electronic properties; Gallium Cu(In; Ga)(Se; S)2; Future challenges; Inorganic semiconductors; Mass production; Material systems; Solution-processed; Solution-processing; State-of-the-art performance Engineering main heading: Solar cells GEOBASE Subject Index: alternative energy; cost analysis; fuel cell; renewable resource; solar power

Towards the growth of SiGeC epitaxial layers for the application in Si solar cells - Wilhelm, Martin; Beck, Felix; Wellmann, Peter (Energy Procedia, 2015)
For the improvement of today's well established silicon solar cells a further significant increase in light conversion efficiency needs the consideration of multi junction aspects. In this context 3C-SiC with a bandgap of 2.3 eV has already been discussed in the past. However, an ideal bandgap match within a silicon tandem cell (Eg(Si) = 1.1 eV) requires a lower bandgap of 1.8 eV which may be reached by the incorporation of Ge in the 3C-SiC lattice. In this work we report on the growth of Si-Ge-C hetero-epitaxial layers on silicon in a MOCVD reactor. A major obstacle in realizing a high Ge-content in the cubic 3C-SiC lattice is the strong trend towards segregation and phase separation in the ternary system. In terms of the atomic diameter, Ge should be more likely to substitute Si in the SiC lattice. However, concerning the excess energy the bond between Ge and C is not supposed to be stable. We will present several growth runs at different precursor flow-rates and different temperatures in the range of 1090 °C - 1330 °C and discuss the appearance of a SiGe phase. In order to detect the different bonds in the layers, especially Ge-C bonds, the samples were investigated by means of Raman-spectroscopy and x-ray diffraction. The Ge concentration was investigated by EDX measurements. No proof for the formation of Ge-C was found so far.
Keywords: 3C-SiC; SiGeC; Epitaxy; Raman; X-Ray diffraction

Towards true contactless outdoor luminescence of silicon photovoltaic modules with inhomogeneous small area excitation source - Doll, Bernd; Kornhas, Jakob; Hepp, Johannes; Buerhop, Claudia; Camus, Christian; Brabec, Christoph; Hauch, Jens (2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC, 2018)
A fast, non-destructive on-site defect detection tool can help to ensure reliable power production of photovoltaic (PV) plants. In particular, it can serve as a cost-effective means for assessing a PV plant's quality and value. Luminescence as a highly sensitive imaging technique can detect local cell damage, series resistance and shunts in solar cells. Typically, outdoor imaging techniques use some sort of modulation of the solar cells' emission. Generally, this requires a physical contact with the module or string. Here, first results obtained from a PL-setup, suitable for fast and fully contactless measurements are presented. The illumination is generated with a small area light source suitable for mobile application
Keywords: reliability; silicon.; photoluminescence; photovoltaic module; defect detection; outdoor characterization; electroluminescence

Towards X-ray in-situ visualization of ammonothermal crystal growth of nitrides - Schimmel, Saskia; Künecke, Ulrike; Baser, Hasan-Hüseyin; Steigerwald, Thomas; Hertweck, Benjamin; Alt, Nicolas; Schlücker, Eberhard; Schwieger, Wilhelm; Wellmann, Peter (Physica Status Solidi (C) Current Topics in Solid State Physics, 2014)
In-situ X-ray visualization of hydrothermal growth of silicalite- 1 is applied as an intermediate step for developing X-ray in-situ visualization techniques for ammonothermal growth of nitride materials. In-situ X-ray imaging of silicalite-1 crystals grown by the bulk material dissolution technique (Shimizu and Hamada, Angew. Chem., Int. Ed. 38, 2725 (1999) [1]) is demonstrated using a stainless steel autoclave equipped with ceramic aluminium oxide windows. Processes in the reaction medium such as local density changes are also visualized. The transferability of the technique to ammonothermal growth of nitrides is evaluated. Furthermore, the applicability of windowless autoclaves for X-ray in-situ monitoring of ammonothermal crystal growth of nitrides is investigated. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: in-situ X-ray imaging; ammonothermal crystal growth; GaN; nitrides

Toward Understanding Space-Charge Limited Current Measurements on Metal Halide Perovskites - Duijnstee, Elisabeth A; Ball, James M; Le Corre, Vincent Marc; Koster, L Jan Anton; Snaith, Henry J; Lim, Jongchul (ACS Energy Letters, 2020)
Metal halide perovskite semiconductors have sprung to the forefront of research into optoelectronic devices and materials, largely because of their remarkable photovoltaic efficiency records above 25% in single-junction devices and 28% in tandem solar cells, achieved within a decade of research. Despite this rapid progress, ionic conduction within the semiconductor still puzzles the community and can have a significant impact on all metal halide perovskite-based optoelectronic devices because of its influence upon electronic and optoelectronic processes. This phenomenon thus also makes the interpretation of electrical characterization techniques, which probe the fundamental properties of these materials, delicate and complex. For example, space-charge limited current measurements are widely used to probe defect densities and carrier mobilities in perovskites. However, the influence of mobile ions upon these measurements is significant but has yet to be considered. Here we report the effect of mobile ions upon electronic conductivity during space-charge limited current measurements of MAPbBr(3) single crystals and show that conventional interpretations deliver erroneous results. We introduce a pulsed-voltage space-charge limited current procedure to achieve reproducible current-voltage characteristics without hysteresis. From this revised pulsed current-voltage sweep, we elucidate a lower bound trap-density value of 2.8 +/- 1.8 x 10(12) cm(-3) in MAPbBr(3) single crystals. This work will lead to more accurate characterization of halide perovskite semiconductors and ultimately more effective device optimization.

Tracking of the growth interface during pvt-growth of SiC boules using a X-ray computed tomography setup - Arzig, Matthias; Salamon, Michael; Uhlmann, Norman; Wellmann, Peter (Materials Science Forum, 2019)
A crystal comprising various polytype switches was analyzed in detail. A very flat temperature profile together with an on axis seed are used to increase the probability for the nucleation of foreign polytypes during growth. Formation of foreign polytype occurred from thin lamella propagating from the edge of the main facet to the crystal rim as well as from 2D nucleation. The evolution of the crystal surface during the growth time is visualized using in-situ XRay computed tomography. Two concurrent growth centers are identified in the beginning of the growth process. Nucleation and lateral growth of foreign polytypes in the main facet area can be traced during the growth process.
CRIS-Team Scopus Importer:2019-09-17
Keywords: Computed tomography; In-situ visualization; Physical vapor transport; Polytype switch; Silicon carbide

Transfer of Heteroepitaxial Grown 3C-SiC Layers for Application in Optical Frequency Combs - Kollmuß, Manuel; Shi, X.; Ou, H.; Wellmann, Peter (Trans Tech Publications Ltd, 2023)
We developed a process for the fabrication of SiCOI stacks which are a suitable platform for optical devices. Starting from 3C-on-Si samples the silicon substrate was removed by wet chemical etching and the remaining 3C-SiC layers were bonded to two different low refractive substrates (Al2O3 and polycrystalline SiC with a 3 µm thick SiO2 layer on top deposited by PECVD). We found that also bonding onto Al2O3 was possible, the stability of the resulting stack wasn´t strong enough for further processing. In contrast mechanical stable SiCOI stacks could be realized using the oxide coated polycrystalline SiC as substrate. Besides the substrate materials three different bonding approaches (hydrophilic, hydrophobic and adhesive bonding using an HSQ resist) as well as multiple process parameters were analyzed with regard to the bonding performance. The best results could be achieved using the adhesive bonding approach with a bonding temperature ≥ 400°C, a process time ≥ 4 h and a bonding pressure of 96 N/cm2.
CRIS-Team Scopus Importer:2023-10-06
Keywords: Bonding; Cubic SiC; Frequency combs; Thin film transfer

Transient absorption spectroscopy studies on polythiophene-fullerene bulk heterojunction organic blend films sensitized with a low-bandgap polymer - Löslein, Heiko; Ameri, Tayebeh; Matt, Gebhard; Koppe, Markus; Egelhaaf, Hans J.; Troeger, Anna; Sgobba, Vito; Guldi, Dirk Michael; Brabec, Christoph (Macromolecular Rapid Communications, 2013)
Recently, the concept of near-infrared sensitization is successfully employed to increase the light harvesting in large-bandgap polymer-based solar cells. To gain deeper insights into the operation mechanism of ternary organic solar cells, a comprehensive understanding of charge transfer-charge transport in ternary blends is a necessity. Herein, P3HT:PCPDTBT:PCBM ternary blend films are investigated by transient absorption spectroscopy. Hole transfer from PCPDTBT-positive polarons to P3HT in the P3HT:PCPDTBT:PCBM 0.9:0.1:1 blend film can be visualized. This process evolves within 140 ps and is discussed with respect to the proposed charge-generation mechanisms. Polythiophene-fullerene bulk heterojunction ternary blend films are investigated by transient absorption spectroscopy. Hole transfer from PCPDTBT-positive polarons to P3HT in the blend film can be visualized. This process evolves within 140 ps and is discussed with respect to the proposed charge-generation mechanisms. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Transparent and Low-Loss Luminescent Solar Concentrators Based on Self-Trapped Exciton Emission in Lead-Free Double Perovskite Nanocrystals - Zdražil, Lukáš; Kalytchuk, Sergii; Langer, Michal; Ahmad, Razi; Pospíšil, Jan; Zmeškal, Oldřich; Altomare, Marco; Osvet, Andres; Zboril, Radek; Schmuki, Patrik; Brabec, Christoph; Otyepka, Michal; et al. (ACS Applied Energy Materials, 2021)
Luminescent solar concentrators (LSCs) are light-harvesting devices that redirect solar light to an edge-attached photovoltaic cell, and thus, they have high potential to be incorporated directly into buildings’ windows to allow for generating electricity. Perovskite nanocrystals (PNCs) are promising materials for LSCs because their enticing optical properties can be engineered to provide a high photoluminescence (PL) quantum yield (QY) and low overlap between absorption and emission spectra. Replacement of toxic, lead-containing perovskites in LSCs by lead-free PNCs, while retaining high optical efficiency of the device, remains the key challenge, which needs to be overcome to build environmentally friendly solar-harvesting platforms. In this work, we use nanocrystals of Bi-doped Cs2Ag0.4Na0.6InCl6 double perovskites with a self-trapped exciton emission to realize for the first time a transparent, low-reabsorption, lead-free perovskite-based LSC. Fabricated 100 cm2 LSCs show an internal optical quantum efficiency of 21.2% with the corresponding internal concentration factor of 2.7. Monte Carlo (MC) ray-tracing simulations identified the loss caused by nonunity PL QY to be the most significant contribution to the overall efficiency loss. The MC simulations also allowed us to estimate the efficiency of 39.4% for 2,500 cm2 LSCs with hypothetical unity PL. These results demonstrate a significant promise held by Bi-doped lead-free PNCs for LSCs.
Keywords: photovoltaics; lead-free perovskites; double perovskites; perovskite nanocrystals; luminescent solar concentrators; Monte Carlo simulations

Trapping effects and surface/interface recombination of carrier recombination in single- or poly-crystalline metal halide perovskites - Lobo, Ntumba; Kawane, Takuya; Matt, Gebhard; Osvet, Andres; Shrestha, Shreetu; Levchuk, Ievgen; Brabec, Christoph; Kanak, Andrii; Fochuk, Petro; Kato, Masashi (Japanese Journal of Applied Physics, 2022)
The lifetime of a carrier is a crucial parameter for solar cell materials, and metal halide perovskite materials are promising for solar cell applications. In this study, we observed carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (mu-PCD) in metal halide perovskite materials: NH3CH3PbI3 (MAPbI(3)), NH3CH3PbBr3 (MAPbBr(3)), and CsPbBr3 with single- and poly-crystalline structures. By comparing the decay curves of TR-PL and mu-PCD, we found trap levels in the band gap for all the materials. We employed two excitation wavelengths for the mu-PCD measurements, and we observed faster mu-PCD signal decays for short wavelength excitation for MAPbBr(3) and CsPbBr3. Additionally, we established that the poly-crystals exhibited faster decay compared with the single crystals for MAPbBr(3) and CsPbBr3. Therefore, we concluded that there are significant contributions of the interface and surface recombination on carrier recombination for MAPbBr(3) and CsPbBr3, but not for MAPbI(3).
CRIS-Team WoS Importer:2022-12-16

Traps and transport resistance are the next frontiers for stable non-fullerene acceptor solar cells - Woepke, Christopher; Goehler, Clemens; Saladina, Maria; Du, Xiaoyan; Nian, Li; Greve, Christopher; Zhu, Chenhui; Yallum, Kaila M.; Hofstetter, Yvonne J.; Becker-Koch, David; Li, Ning; Heumüller, Thomas; et al. (Nature Communications, 2022)
Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.
CRIS-Team Scopus Importer:2022-07-15

Triphenylamine-Based Push-Pull Molecule for Photovoltaic Applications: From Synthesis to Ultrafast Device Photophysics - Kozlov, Oleg V.; Liu, Xiaomeng; Luponosov, Yuriy N.; Solodukhin, Alexander N.; Toropynina, Victoria Y.; Min, Jie; Buzin, Mikhail I.; Peregudova, Svetlana M.; Brabec, Christoph; Ponomarenko, Sergei A.; Pshenichnikov, Maxim S. (Journal of Physical Chemistry C, 2017)
Small push-pull molecules attract much attention as prospective donor materials for organic solar cells (OSCs). By chemical engineering, it is possible to combine a number of attractive properties such as broad absorption, efficient charge separation, and vacuum and solution processabilities in a single molecule. Here we report the synthesis and early time photophysics of such a molecule, TPA-2T-DCV-Me, based on the triphenylamine (TPA) donor core and dicyanovinyl (DCV) acceptor end group connected by a thiophene bridge. Using time-resolved photoinduced absorption and photoluminescence, we demonstrate that in blends with [70]PCBM the molecule works both as an electron donor and hole acceptor, thereby allowing for two independent channels of charge generation. The charge-generation process is followed by the recombination of interfacial charge transfer states that takes place on the subnanosecond time scale as revealed by time-resolved photoluminescence and nongeminate recombination as follows from the OSC performance. Our findings demonstrate the potential of TPA-DCV-based molecules as donor materials for both solution-processed and vacuum-deposited OSCs. (Figure Presented). © 2017 American Chemical Society.
Keywords: Charge transfer; Molecules; Organic solar cells; Photoluminescence; Solar cells; Vacuum applications; Charge separations; Independent channels; Interfacial charge transfer; Photoinduced absorption; Photovoltaic applications; Push-pull molecules; Subnanosecond time scale; Time-resolved photoluminescence; Synthesis (chemical)

Tunable doping in PbS nanocrystal field-effect transistors using surface molecular dipoles - Nugraha, Mohamad I.; Matsui, Hiroyuki; Bisri, Satria Z.; Sytnyk, Mykhailo; Heiß, Wolfgang; Loi, Maria A.; Takeya, Jun; Heiß, Wolfgang (APL Materials, 2016)
We study the effect of self-assembled monolayer (SAM) treatment of the SiO dielectric on the electrical characteristics of PbS transistors. Using SAMs, we observe threshold voltage shifts in the electron transport, allowing us to tune the electrical properties of the devices depending on the SAM molecule used. Moreover, the use of a specific SAM improves the charge carrier mobility in the devices by a factor of three, which is attributed to the reduced interface traps due to passivated silanol on the SiO surface. These reduced traps confirm that the voltage shifts are not caused by the trap states induced by the SAMs.

Tuning Electrical and Optical Properties of Transparent Conductive Thin Films Using ITO and ZnO Nanoparticles, Sol-Gel-ZnO and Ag Nanowires - Schuster, Matthias; Groß, Stephanie; Roider, Florian; Maksimenko, Ilja; Wellmann, Peter (International Journal of Nanoparticles and Nanotechnology, 2017)
The progress in the research on a wet-chemical or nanoparticle derived absorber layer for solar cells leads to the fabrication of a printable transparent front electrode. This eliminates the need for vacuum technology, when the vacuum chain is already breached for the absorber deposition. The objective of this work is the manufacturing and characterization of a vacuum-free, printable, transparent front electrode on the basis of conductive metal oxides like ITO and ZnO, while reducing Indium consumption and annealing temperature or duration. Nanoparticles as well as a sol-gel process were used to deposit transparent and conductive Aluminum Doped Zinc Oxide (AZO) thin films on glass substrates. Furthermore, the influence of ion-implantation-doping after layer deposition was examined. Different temperature treatments were applied after layer deposition or ion-implantation respectively to tune the conductivity of the thin films. For characterization of the thin-films fourpoint-probe measurements, white-light-interferometry, UV/VIS spectroscopy, and (FE) SEM were used. The sol-gel-layers achieved a conductivity of 314 S/cm whereas the nanoparticle derived ZnO layers only reached 1.15 - 20 S/cm. Finally, the application of silver nanowires resulted in a low sheet resistance of 5.8 Ω/□ which is even lower than for state-of-the-art sputtered AZO-layers (15 Ω/□) and allows low annealing temperatures of only 140 °C.
Keywords: Transparent conductive film; ITO & ZnO nanoparticles; Sol-Gel ZnO:(In; Al); Silver nanowires (Ag-NW)

Tuning of electronic states in self-assembled InAs quantum dots using an ion implantation technique - Wellmann, Peter; Schoenfeld, W.V.; Garcia, J.M.; Petroff, P.M. (Journal of Electronic Materials, 1998)
We report the tunability of up to 150 meV of the ground state transition of selfassembled InAs quantum dots (QDs) using Mn ion implantation and subsequent annealing. Because of the exciton localization in the quantum dots, the photoluminescence efficiency (T = 12K) of the quantum dot transition remains at 80% of its original value after implantation with a Mn dose of 1 ´ 1013 cm–2 ions. Strong luminescence still remains at room temperature. At a high implantation dose (1 ´ 1015 cm–2) and rapid thermal annealing (700°C for 60s) about 25% of the QD luminescence intensity is recovered at T = 12K.<br
Keywords: InAs quantum dots; Ion implantation

Tuning the emission colour by manipulating terbium-terbium interactions: Terbium doped aluminum nitride as an example system - Benz, Felix; Strunk, Horst Paul; Schaab, Jakob; Künecke, Ulrike; Wellmann, Peter (Journal of Applied Physics, 2013)
Terbium-terbium interactions in terbium doped semiconductors and insulators may lead to the so-called cross-relaxation process, which increases the D-5(4) (green) emission of the terbium ions at the cost of the D-5(3) (blue) luminescence intensity. This effect can generally be reduced by increasing the distance between an excited ion and the nearest ion in the ground state. A straightforward measure is to use a specimen with a decreased terbium concentration. The alternative is to increase the intensity of the excitation (either by photons or electrons) and thereby to reduce the population of terbium ions in the ground state. This paper works this process out with the example of AlN:Tb on the basis of a model and respective experimental results. As will be seen, stronger excitation causes in essence more Tb ions to be excited, thus less ions in the ground state which increases the distance between an excited and the nearest ground state ions. This hinders energy transfer between the terbium ions and thus counteracts the cross-relaxation process. The advantage of changing the excitation intensity lies in the possibility to deliberately shift the apparent colour of the Tb luminescence from a single specimen between green and blue. (C) 2013 AIP Publishing LL
Keywords: Aluminiumnitride; AlN; Tb doping; Tb-Tb interactions; cross relaxation; luminescence; rare earth concentration; excitation intensity

Tuning the Localized Surface Plasmon Resonance in Cu2-xSe Nanocrystals by Postsynthetic Ligand Exchange - Balitskii, Olexiy A.; Sytnyk, Mykhailo; Stangl, Julian; Primetzhofer, Daniel; Groiss, Heiko; Heiß, Wolfgang (ACS Applied Materials and Interfaces, 2014)
Nanoparticles exhibiting localized surface plasmon resonances (LSPR) are valuable tools traditionally used in a wide field of applications including sensing, imaging, biodiagnostics and medical therapy. Plasmonics in semiconductor nanocrystals is of special interest because of the tunability of the carrier densities in semiconductors, and the possibility to couple the plasmonic resonances to quantum confined excitonic transitions. Here, colloidal Cu2-xSe nanocrystals were synthesized, whose composition was shown by Rutherford backscattering analysis and electron dispersive X-ray spectroscopy, to exhibit Cu deficiency. The latter results in p-type doping causing LSPRs, in the present case around a wavelength of 1100 nm, closely matching the indirect band gap of Cu2-xSe. By partial exchange of the organic ligands to specific electron trapping or donating species the LSPR is fine-tuned to exhibit blue or red shifts, in total up to 200 nm. This tuning not only provides a convenient tool for post synthetic adjustments of LSPRs to specific target wavelength but the sensitive dependence of the resonance wavelength on surface charges makes these nanocrystals also interesting for sensing applications, to detect analytes dressed by functional groups.
Keywords: colloidal nanocrystals; plasmon resonance; ligand exchange; interfacial charge transfer

Tuning the Magnetic Properties of Metal Oxide Nanocrystal Heterostructures by Cation Exchange - Sytnyk, Mykhailo; Kirchschlager, Raimund; Bodnarchuk, Maryna I.; Primetzhofer, Daniel; Kriegner, Dominik; Enser, Herbert; Stangl, Julian; Bauer, Peter; Voith, Michael; Hassel, Achim Walter; Krumeich, Frank; Ludwig, Frank; et al. (Nano Letters, 2013)
For three types of colloidal magnetic nanocrystals, we demonstrate that postsynthetic cation exchange enables tuning of the nanocrystal's magnetic properties and achieving characteristics not obtainable by conventional synthetic routes. While the cation exchange procedure, performed in solution phase approach, was restricted so far to chalcogenide based semiconductor nanocrystals, here ferrite-based nanocrystals were subjected to a Fe2+ to Co2+ cation exchange procedure. This allows tracing of the compositional modifications by systematic and detailed magnetic characterization. In homogeneous magnetite nanocrystals and in gold/magnetite core shell nanocrystals the cation exchange increases the coercivity field, the remanence magnetization, as well as the superparamagnetic blocking temperature. For core/shell nanoheterostructures a selective doping of either the shell or predominantly of the core with Co2+ is demonstrated. By applying the cation exchange to FeO/CoFe2O4 core/shell nanocrystals the Neel temperature of the core material is increased and exchange-bias effects are enhanced so that vertical shifts of the hysteresis loops are obtained which are superior to those in any other system.
Keywords: magnetic nanoctystals; cation exchange; exchange bias; nanoheterostructures; ferrites

Tuning the Properties of CZTS Films by Controlling the Process Parameters in Cost-Effective Non-vacuum Technique - Abdelhaleem, Soraya; Hassanien, A.E.; Ahmad, Rameez; Schuster, Matthias; Ashour, A.H.; Distaso, Monica; Peukert, Wolfgang; Wellmann, Peter (Journal of Electronic Materials, 2018)
Highly dispersive Cu2ZnSnS4 (CZTS) nanoparticles were successfully synthesized by a simple solvothermal route. A low cost, non-vacuum method was used to deposit CZTS nanoparticle ink on glass substrates by a doctor blade process followed by selenization in a tube furnace to form Cu2ZnSn (S,Se)4 (CZTSSe) layers. Different selenization conditions and particle concentrations were considered in order to improve the crystallinity and surface morphology; the annealing temperature was varied between 400C and 550C and the annealing time was varied between 5 min and 20 min in a seleniumnitrogen atmosphere. The influence of annealing conditions on structural, compositional, optical and electrical properties of CZTSSe thin films was studied. An improvement in the structural and surface morphology was observed with increasing of annealing temperature (up to 500C). An enhancement in the crystallinity and surface morphology were observed for thin films annealed for 10–15 min. Absorption study revealed that the band gap energy of as-deposited CZTS thin film was approximately 1.43 eV, while for CZTSSe thin films it ranged from 1.15 eV to 1.34 eV at different annealing temperatures, and from 1.33 eV to 1.38 eV for different annealing times.<br
Keywords: CZTSSe; selenization; solvothermal; thin films; doctor blade; resistivity

Two similar near-infrared (IR) absorbing benzannulated AZA-bodipy dyes as near-ir sensitizers for ternary solar cells - Min, Jie; Ameri, Tayebeh; Gresser, Roland; Lorenz-Rothe, Melanie; Baran, Derya; Troeger, Anna; Sgobba, Vito; Leo, Karl; Riede, Moritz; Guldi, Dirk Michael; Brabec, Christoph (ACS Applied Materials and Interfaces, 2013)
Ternary composite inverted organic solar cells based on poly(3-hexylthiophen-2,5-diyl) (P3HT) and phenyl-C-butyric acid methyl ester (PCBM) blended with two different near-infrared absorbing benzannulated aza-BODIPY dyes, difluoro-bora-bis-(1-phenyl-indoyl)-azamethine (1) or difluoro-bora-bis-(1-(5-methylthiophen)-indoyl)-azamethine (2), were constructed and characterized. The amount of these two aza-BODIPY dyes, within the P3HT and PCBM matrix, was systematically varied, and the characteristics of the respective devices were recorded. Although the addition of both aza-BODIPY dyes enhanced the absorption of the blends, only the addition of 1 improved the overall power conversion efficiency (PCE) in the near-infrared (IR) region. The present work paves the way for the integration of near-infrared absorbing aza-BODIPY derivatives as sensitizers in ternary composite solar cells. © 2013 American Chemical Society.

Ultrafast screening method for assessing the photostability of thin-film solar cells - Vetter, Andreas; Burlafinger, Klaus; Brabec, Christoph (Electrical Engineering, 2016)
Commercial success of optoelectronical devices such as solar cells depend strongly on lifetime next to production costs and power conversion efficiency. While a steep decrease in production costs and significant increase in efficiency have been achieved, lifetime still plays often a limiting factor, in particular, in case of highly innovative new device types. Lifetime tests measuring the stability of devices without external influences (i. p. water) generally take very long time due to the required long lifetime of optoelectronical devices. We established a novel accelerated lifetime test (ALT) setup which may increase the test speed by a factor of several hundred. We verified the setup and the applicability of our ALT measurement routine with an experiment on a well-studied innovative thin-film solar cell type (P3HT:PCBM).
Keywords: ALT; Optoelectronics; Organic photovoltaics; PV; Solar cell; Thin film

Ultra-fine metal particles dispersed on single-walled carbon nanotubes for energy devices - Ishii, Yosuke; Ishikawa, Sae; Yamada, Itta; Kondo, Kohei; Jindo, Shinya; Kawasaki, Shinji; Hattori, Yoshiyuki; Mashkov, Oleksandr; Heiß, Wolfgang (Journal of Materials Science, 2022)
We prepared metal nano-particles deposited on SWCNTs by heat-treatment of SWCNTs encapsulating metal complex (Ni(acac)2, Cu(acac)2) molecules inside their hollow cores. It was confirmed by synchrotron radiation x-ray absorption spectra that the obtained nano-particles were metallic Ni and Cu. Electron microscopic measurements revealed that the diameters of most of the metal particles are less than 10 nm and that they were well dispersed on SWCNTs even in the bundles. Ni metal particles on SWCNTs were converted to Ni(OH)2 particles by electrochemical treatment in 1 M KOH aqueous solution. We investigated the Ni-MH battery electrode properties of the obtained Ni(OH)2/SWCNT sample. We expect Ni(OH)2/SWCNT to work as high-power battery electrode, because the reversible charge/discharge capacity of Ni(OH)2/SWCNT did not change much with increasing the scan rate from 2 to 1000 mV/s. Cu/SWCNT worked as a CO2 photo-reduction catalyst by combing it with light-absorbing semiconductors. In the photo-catalyst, Cu works as a metal co-catalyst to enhance the reaction efficiency. We used inorganic semiconductor TiO2 and organic semiconductor epindolidine (EPI). It was found that both Cu/SWCNT/TiO2 and Cu/SWCNT/EPI can work as CO2 photo-reduction catalyst. The external CO2 to CO quantum yield of Cu/SWCNT/EPI catalyst under the simulated solar light (AM1.5G, 100 mW/cm2) was 2.2 × 10−4%. Graphical abstract: [Figure not available: see fulltext.]
CRIS-Team Scopus Importer:2022-02-04

Ultra low band gap α,β-unsubstituted BODIPY-based copolymer synthesized by palladium catalyzed cross-coupling polymerization for near infrared organic photovoltaics - Squeo, Benedetta M.; Gasparini, Nicola; Ameri, Tayebeh; Palma-Cando, Alex; Allard, Sybille; Gregoriou, Vasilis G.; Brabec, Christoph; Scherf, Ullrich; Chochos, Christos L. (Journal of Materials Chemistry A, 2015)
A new ultra low band gap (LBG) α,β-unsubstituted BODIPY-based conjugated polymer has been synthesized by conventional cross coupling polymerization techniques (Stille cross coupling) for the first time. The polymer exhibits a panchromatic absorption spectrum ranging from 300 nm to 1100 nm and an optical band gap (Eoptg) of 1.15 eV, suitable for near infrared (NIR) organic photovoltaic applications as electron donor. Preliminary power conversion efficiency (PCE) of 1.1% in polymer : [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) 1 : 3 weight ratio bulk heterojunction (BHJ) solar cells has been achieved, demonstrating very interesting and promising photovoltaic characteristics, such as good fill factor (FF) and open circuit voltage (Voc). These results showing that by the proper chemical design, new α,β-unsubstituted BODIPY-based NIR copolymers can be developed in the future with suitable energy levels matching those of PC71BM towards more efficient NIR organic photovoltaics (OPVs).

Ultrastable Single-component Material Devices: the Next Frontier for Organic Solar Cells - He, Yakun; Bäuerle, Peter; Li, Weiwei; Li, Ning; Brabec, Christoph (nanoGe, 2022)

Ultrastable single-component organic solar cells: the next frontier towards industrial viability - He, Yakun; Brabec, Christoph; Heumüller, Thomas; Wortmann, Jonas; Hanisch, Benedict; Aubele, Anna; Li, Ning; Feng, Guitao; Jiang, Xudong; Li, Weiwei; Bäuerle, Peter; Lucas, Sebastian (Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22), 2022)
Power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) organic solar cells (OSCs) continue increasing towards the 20% milestone mainly due to the emerging non-fullerene acceptors (NFAs). While significant efforts have been devoted to efficiency analysis, important factors for industrial application are mostly neglected, such as photostability and the cost potential. Single-component organic solar cells (SCOSCs) employing materials with donor and acceptor moieties chemically bonded within one molecule or polymer, successfully overcome the immiscibility between donor and acceptor as well as the resultant self-aggregation under external stress [1]. To inspire a broader interest, in this work, the industrial figure of merit (i-FOM) of OSCs are calculated and analyzed, which includes PCE, photostability, and synthetic complexity (SC) index [2]. Based on the notable advantages of SCOSCs over the correspondent BHJ OSCs, especially the enhanced stability and simplified film processing, we systematically compare the i-FOM values of BHJ OSCs and the corresponding SCOSCs. SCMs exhibit overall much higher i-FOM values compared with the BHJ OSCs, and the highest value reaches 0.3, which is comparable with (or higher than) the ones of high-efficiency BHJ OSCs. With the increase of efficiency, SCOSCs possess the further potential to a higher i-FOM value. Among all factors, the synthetic complexity of SCOSCs is slightly higher than that of the corresponding BHJ OSCs due to the extra synthetic step for connecting donor and acceptor moieties. Nevertheless, the covalent bonding structure of SCMs helps overcome the immiscibility between D and A for proper nanophase separation and consequently a decent PCE, in comparison with the large-scale phase separation for the corresponding BHJ systems. More importantly, the higher i-FOM of SCOSCs is mostly ascribed to the substantially enhanced stability compared with BHJ [3]. SCOSCs based on dyad 1 exhibit surprisingly high photostability under concentrated light (7.5 suns and 30 suns), corresponding to an almost unchanged device stability up to ten thousand hours under 1-sun illumination. For realizing industrial application, SCOSCs have to give a high efficiency comparable to the current high-efficiency BHJ OSCs, while BHJ should be developed in the direction of less complicated synthesis. With joint efforts of researchers from multidiscipline, SCOSCs will see continuing progress in efficiency, reaching an i-FOM value enough for industrial application. <br

Ultra-stable single component organic solar cells under thermal and/or illumination pressure: the next superior organic photovoltaics? - He, Yakun; Baeuerle, Peter; Li, Weiwei; Heumüller, Thomas; Li, Ning; Brabec, Christoph (13th Conference on Hybrid and Organic Photovoltaics (HOPV21), 2021)
Stability issue is the core restriction for the application of organic solar cells (OSCs). However, state-of-the-art OSCs based on bulk-heterojunction concept suffer from stability problems caused by the severe morphological changes upon thermal or illumination stress [1]. Single-component concept, enabled by the covalently-bonded structure with donor and acceptor in one molecule, presents attractive advantages such as a simplification of device fabrication and a stabilization of microstructure [2]. Recently, with the rapid improvement of efficiency from 2-3% to 8.4% for single-component organic solar cells (SCOSCs), the reports on their stability however are scanty. In this work, we for the first time systematically investigated the stability under thermal and illumination stress of a series of SCOSCs based on polymeric (SCP3, PBDBPBI-Cl) and molecular (Dyad1, 2, 3, 4) materials. Under thermal pressure, double-cable polymer-based SCOSCs exhibited excellent thermal stability with no degradation at 90 oC for 3000 hours, partially attributed to the high temperatures (around 200 oC) for post-treating active layer during device fabrication. Such rather high annealing temperatures are plausible to create microstructure arrangements which are in thermodynamic equilibrium after cooling down. Furthermore, without precedent, we studied the thermal stability comparison among a series of SCOSCs based on D-A small molecules (Dyad1, 2 and 3) with the same donor and acceptor units but differently long alkyl space linkers. Since macroscopic diffusion of molecules is excluded in these dyads, the length of the spacer can only provide the necessary flexibility for sub-nm rearrangements caused by thermal stress. Interestingly, dyads showed a distinctly different behavior, Dyad1 with the shortest linker exhibiting the highest thermal stability, while Dyad3 with the longest linker showing the worst thermal stability [3]. This highlights the need for further in-depth studies on the dual importance of spacer length for performance and stability. Moreover, Dyad1-based SCOSCs exhibited extraordinary illumination stability, retaining 98% of the initial PCE under concentrated light equal to 7.5-suns for over 1000 hours, which could be among the highest illumination stability for solution-processed OSCs. Based on the outstanding stability, SCOSCs could be an ideal candidate to study the ultimate stability under extremely rugged accelerating conditions such as high temperature and concentrated light. Since the morphological evolution is excluded, SCOSCs could serve as a model system to selectively study interface degradation. In the near future, SCOSCs will see a prospective renaissance with 10% efficiency and 20 years lifetime for industrial applications.

Understanding and Controlling the Evolution of Nanomorphology and Crystallinity of Organic Bulk-Heterojunction Blends with Solvent Vapor Annealing - Harreiß, Christina; Langner, Stefan; Wu, Mingjian; Berlinghof, Marvin; Rechberger, Stefanie; Will, Johannes; Conroy, Michele; Bangert, Ursel; Unruh, Tobias; Brabec, Christoph; Spiecker, Erdmann (Solar RRL, 2022)
Solvent vapor annealing (SVA) has been shown to significantly improve the device performance of organic bulk-heterojunction solar cells, yet the mechanisms linking nanomorphology, crystallinity of the active layer, and performance are still largely missing. Here, the mechanisms are tackled by correlating the evolution of nanomorphology, crystallinity, and performance with advanced transmission electron microscopy methods systematically. Model system of DRCN5T:PC71BM blends are SVA treated with four solvents differing in their donor and acceptor solubilities. The choice of solvent drastically influences the rate at which the maximum device efficiency establishes, though similar values can be achieved for all solvents. The donor solubility is identified as a key parameter that controls the kinetics of diffusion and crystallization of the blend molecules, resulting in an inverse relationship between optimal annealing time and donor solubility. For the highest efficiency, optimum domain size and single-crystalline nature of DRCN5T fibers are found to be crucial. Moreover, the π–π stacking orientation of the crystallites is directly revealed and related to the nanomorphology, providing insight into the charge carrier transport pathways. Finally, a qualitative model relating morphology, crystallinity, and device efficiency evolution during SVA is presented, which may be transferred to other light-harvesting blends.
Created from Fastlane, Scopus look-up
Keywords: crystallinity; EF-ED; EFTEM; nanomorphology; organic BHJ solar cells; small molecules; SVA

Understanding Causalities in Organic Photovoltaics Device Degradation in a Machine-Learning-Driven High-Throughput Platform - Liu, Chao; Lüer, Larry; Le Corre, Vincent Marc; Forberich, Karen; Weitz, Paul; Heumüller, Thomas; Du, Xiaoyan; Wortmann, Jonas; Zhang, Jiyun; Wagner, Jerrit; Ying, Lei; Hauch, Jens; et al. (Advanced Materials, 2023)
Organic solar cells (OSCs) now approach power conversion efficiencies of 20%. However, in order to enter mass markets, problems in upscaling and operational lifetime have to be solved, both concerning the connection between processing conditions and active layer morphology. Morphological studies supporting the development of structure–process–property relations are time-consuming, complex, and expensive to undergo and for which statistics, needed to assess significance, are difficult to be collected. This work demonstrates that causal relationships between processing conditions, morphology, and stability can be obtained in a high-throughput method by combining low-cost automated experiments with data-driven analysis methods. An automatic spectral modeling feeds parametrized absorption data into a feature selection technique that is combined with Gaussian process regression to quantify deterministic relationships linking morphological features and processing conditions with device functionality. The effect of the active layer thickness and the morphological order is further modeled by drift–diffusion simulations and returns valuable insight into the underlying mechanisms for improving device stability by tuning the microstructure morphology with versatile approaches. Predicting microstructural features as a function of processing parameters is decisive know-how for the large-scale production of OSCs.
CRIS-Team Scopus Importer:2023-04-28
Keywords: device stability; Gaussian process regression (GPR) prediction; linear Pearson correlations; machine learning; organic solar cells

Understanding Dark Current-Voltage Characteristics in Metal-Halide Perovskite Single Crystals - Duijnstee, Elisabeth A; Le Corre, Vincent Marc; Johnston, Michael B.; Koster, L Jan Anton; Lim, Jongchul; Snaith, Henry J. (Physical Review Applied, 2021)
Hybrid halide perovskites have great potential for application in optoelectronic devices. However, an understanding of some basic properties, such as charge-carrier transport, remains inconclusive, mainly due to the mixed ionic and electronic nature of these materials. Here, we perform temperature-dependent pulsed-voltage space-charge-limited current measurements to provide a detailed look into the electronic properties of methylammonium lead tribromide (MAPbBr(3)) and methylammonium lead triiodide (MAPbI(3)) single crystals. We show that the background carrier density in these crystals is orders of magnitude higher than that expected from thermally excited carriers from the valence band. We highlight the complexity of the system via a combination of experiments and drift-diffusion simulations and show that different factors, such as thermal injection from the electrodes, temperature-dependent mobility, and trap and ion density, influence the free-carrier concentration. We experimentally determine effective activation energies for conductivity of (349 +/- 10) meV for MAPbBr3 and (193 +/- 12) meV for MAPbI(3), which includes the sum of all of these factors. We point out that fitting the dark current density-voltage curve with a drift-diffusion model allows for the extraction of intrinsic parameters, such as mobility and trap and ion density. From simulations, we determine a charge-carrier mobility of 12.9 cm(2)/Vs, a trap density of 1.52 x 10(13) cm(-3), and an ion density of 3.19 x 10(12) cm(-3) for MAPbBr(3) single crystals. Insights into charge-carrier transport in metal-halide perovskite single crystals will be beneficial for device optimization in various optoelectronic applications.

Understanding degradation mechanisms of perovskite solar cells due to electrochemical metallization effect - Wang, Hai Qiao; Wang, Sunan; Chen, Lijun; Yin, Zhipeng; Mei, Sikai; Zhong, Ying; Yao, Yirong; Li, Ning; Wang, Jianhao; Song, Weijie (Solar Energy Materials and Solar Cells, 2021)
Operational stability is becoming one of the most crucial parameters for commercialization of perovskite solar cells (PSCs). However the stability issue of PSCs is currently far from being resolved due to complicated and still unclear degradations. In this work, we systematically demonstrate the degradation of metal cathode PSCs under operation conditions. Discussions about influence of intrinsic factors i.e. light illumination, voltage and current, on device degradation are conducted. It is concluded that metal cathode stripping/plating behavior (electrochemical metallization effect) due to current together with perovskite degradation could dominate the device degradation. It is deduced that electrochemical cell in the PSC system could be formed by metal/counter electrodes and perovskite electrolyte. Both cells accelerate degradation of metal electrode and perovskite in working conditions, hence device degradation. These insights into the degradation and mechanisms can help further understand the working principle and solve the instability problem of perovskite-based optoelectronic devices.
CRIS-Team Scopus Importer:2021-08-13
Keywords: Electrochemical metallization effect; Metal electrode; Perovskite solar cell; Stability

Understanding the correlation and balance between the miscibility and optoelectronic properties of polymer-fullerene solar cells - Zhang, Chaohong; Langner, Stefan; Mumyatov, Alexander V.; Anokhin, Denis V.; Min, Jie; Perea, Jose Dario; Gerasimov, Kirill L.; Osvet, Andres; Ivanov, Dimitri A.; Troshin, Pavel; Li, Ning; Brabec, Christoph (Journal of Materials Chemistry A, 2017)
Organic photovoltaics is one of the most promising technologies for sustainable green energy supply. Because of their high electron affinity and superior electron-transporting ability, fullerene-based materials are deemed as very strong electron-accepting components in organic solar cells. However, the most widely used fullerene-based acceptors, such as phenyl-C61-butyric acid methyl ester, exhibit limited microstructural stability and unsatisfactory thermal stability owing to their insufficient compatibility with organic donors. Here, we in-depth investigate the carrier dynamics along with structural evolution and analyze the acceptor loadings in optimized bulk-heterojunction (BHJ) solar cells as a function of the polymer-fullerene miscibility. The polymer-fullerene miscibility has more influential effects than the crystallinity of single components on the optimized acceptor:donor ratio in polymer-fullerene solar cells. The findings demonstrated in this work suggest that the balance between the miscibility of BHJ composites and their optoelectronic properties has to be carefully considered for future development and optimization of OPV solar cells based on BHJ composites. Miscibility is proposed in addition to crystallinity as a further design criterion for long lived and efficient solar cells. © 2017 The Royal Society of Chemistry.
Keywords: Engineering controlled terms: Butyric acidElectron affinityFullerenesHeterojunctionsOrganic solar cellsPolymer solar cellsPolymersSolubility Bulk heterojunction (BHJ)Electron transportingHigh electron affinitiesInfluential effectsMicrostructural stabilityOptoelectronic propertiesOrganic photovoltaicsStructural evolution Engineering main heading: Solar cells

Understanding the Limitations of Charge Transporting Layers in Mixed Lead–Tin Halide Perovskite Solar Cells - Zhang, Kaicheng; Forberich, Karen; Lüer, Larry; García Cerrillo, José; Meng, Wei; Du, Xiaoyan; Le Corre, Vincent Marc; Zhao, Yicheng; Niu, Tianqi; Xue, Qifan; Koster, L Jan Anton; Li, Ning; et al. (Advanced Energy and Sustainability Research, 2021)

Understanding the Microstructure Formation of Polymer Films by Spontaneous Solution Spreading Coating with a High-Throughput Engineering Platform - Wang, Rong; Lüer, Larry; Langner, Stefan; Heumüller, Thomas; Forberich, Karen; Zhang, Heyi; Hauch, Jens; Li, Ning; Brabec, Christoph (Chemsuschem, 2021)
An important step of the great achievement of organic solar cells in power conversion efficiency is the development of low-band gap polymer donors, PBDB−T derivatives, which present interesting aggregation effects dominating the device performance. The aggregation of polymers can be manipulated by a series of variables from a materials design and processing conditions perspective; however, optimization of film quality is a time- and energy-consuming work. Here, we introduce a robot-based high-throughput platform (HTP) that is offering automated film preparation and optical spectroscopy thin-film characterization in combination with an analysis algorithm. PM6 films are prepared by the so-called spontaneous film spreading (SFS) process, where a polymer solution is coated on a water surface. Automated acquisition of UV/Vis and photoluminescence (PL) spectra and automated extraction of morphological features is coupled to Gaussian Process Regression to exploit available experimental evidence for morphology optimization but also for hypothesis formulation and testing with respect to the underlying physical principles. The integrated spectral modeling workflow yields quantitative microstructure information by distinguishing amorphous from ordered phases and assesses the extension of amorphous versus the ordered domains. This research provides an easy to use methodology to analyze the exciton coherence length in conjugated semiconductors and will allow to optimize exciton splitting in thin film organic semiconductor layers as a function of processing.
CRIS-Team Scopus Importer:2021-07-16
Keywords: Gaussian process regression; high-throughput engineering; microstructure morphology; organic photovoltaics; spontaneous film spreading

Universal Current Losses in Perovskite Solar Cells Due to Mobile Ions - Thiesbrummel, Jarla; Le Corre, Vincent Marc; Peña‐Camargo, Francisco; Perdigón-Toro, Lorena; Lang, Felix; Yang, Fengjiu; Grischek, Max; Gutierrez‐Partida, Emilio; Warby, Jonathan; Farrar, Michael D; Mahesh, Suhas; Caprioglio, Pietro; et al. (Advanced Energy Materials, 2021)
Efficient mixed metal lead-tin halide perovskites are essential for the development of all-perovskite tandem solar cells, however they are currently limited by significant short-circuit current losses despite their near optimal bandgap (approximate to 1.25 eV). Herein, the origin of these losses is investigated, using a combination of voltage dependent photoluminescence (PL) timeseries and various charge extraction measurements. It is demonstrated that the Pb/Sn-perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted PL from the device rises on the exact same timescales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping-induced electronic charges but by the movement of mobile ions toward the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. Finally, these findings are generalized to lead-based perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism.
Keywords: current losses; mobile ions; non-radiative recombination; perovskite solar cells; Sn perovskite; transient photoluminescence

Unraveling Dual Operational Mechanisms in an Air-Stable All Inorganic Perovskite for Nonvolatile Memory and Neuromorphic Computing - Xie, Zhiqiang; Zhang, Difei; Cheng, Long; Li, Chaohui; Elia, Jack; Wu, Jianchang; Tian, Jingjing; Chen, Lijun; Loi, Maria Antonietta; Osvet, Andres; Brabec, Christoph (ACS Energy Letters, 2024)
Two-terminal drift memristors (nonvolatile) are widely employed to emulate biological synaptic functionalities in neuromorphic architectures. However, reliable emulations of synaptic dynamics can only be achieved through the integration of their counterparts, diffusive memristors. Moreover, the combination of drift and diffusive memristors represents a desirable approach to address the escalating demands posed by the increasing complexity of neuromorphic computing frameworks, which are still in their nascent stages. Accordingly, an air-stable inorganic perovskite memristor (RbPbI3) is demonstrated with adjustable drift and diffusive modes. By employing an electroforming process, the drift-type devices demonstrate bipolar resistive switching with a large ON/OFF ratio (102), stable endurance (2000 cycles), long retention (1.2 × 105 s), and robust air stability. In contrast, diffusive-type devices, without an electroforming process, effectively emulate synaptic behaviors, including paired-pulse facilitation, long-term potentiation/depression, and spike-timing-dependent plasticity. Additionally, experimental data are utilized to train neural networks constructed with perovskite artificial synapses on image classification tasks. The results demonstrate accuracies of 89.24% (MNIST) and 79.10% (Fashion-MNIST) under supervised learning, closely approximating their theoretical values.
CRIS-Team Scopus Importer:2024-03-01

Unraveling the Charge-Carrier Dynamics from the Femtosecond to the Microsecond Time Scale in Double-Cable Polymer-Based Single-Component Organic Solar Cells - He, Yakun; Wang, Bingzhe; Lüer, Larry; Feng, Guitao; Osvet, Andres; Heumüller, Thomas; Liu, Chao; Li, Weiwei; Guldi, Dirk Michael; Li, Ning; Brabec, Christoph (Advanced Energy Materials, 2021)
Single-component organic solar cells (SCOSCs) have witnessed great improvement during the last few years with the champion efficiency jumping from the previous 2-3% to currently 6-11% for the representative material classes. However, the photophysics in many of these materials has not been sufficiently investigated, lacking essential information regarding charge-carrier dynamics as a function of microstructure, which is highly demanded for a better understanding and potential guidance for further improvements. In this work, for the first time, the charge-carrier dynamics of a representative double-cable polymer, which achieves efficiencies of over 6% as an active layer in SCOSCs, is investigated across seven orders of magnitude in time scale, from fs-ps charge generation to ns-mu s charge recombination processes. Specific emphasis is placed on understanding the impact of thermal post-treatment on the charge dissociation and recombination dynamics. Annealing the photoactive layer at 230 degrees C results in the highest photovoltaic performance because of efficient charge generation in parallel to suppressed recombination. This work intends to present a complete picture of the charge-carrier dynamics in SCOSCs using the representative double-cable polymer PBDBPBI-Cl.
CRIS-Team WoS Importer:2021-12-31

Unraveling the Complex Nanomorphology of Ternary Organic Solar Cells with Multimodal Analytical Transmission Electron Microscopy - Rechberger, Stefanie; Gasparini, Nicola; Singh, Ranbir; Kim, Min; Chochos, Christos L.; Gregoriou, Vasilis G.; Cho, Kilwon; Brabec, Christoph; Ameri, Tayebeh; Spiecker, Erdmann (Solar RRL, 2020)
Elucidating the complex materials distribution in the active layers of ternary organic solar cells is one of the greatest challenges in the field of organic photovoltaics. Knowledge of the nanomorphology is key to understanding photophysical processes (e.g., charge separation, adjustment of the recombination mechanism, and suppression of the radiationless and energetic losses) and thus improving the device performance. Herein, for the first time, the successful discrimination and spatial mapping of the active layer components of a ternary organic solar cell are demonstrated using analytical transmission electron microscopy. The material distribution of all three organic components is successfully visualized by multimodal imaging using complementary electron energy loss signals. A complete picture of the morphological aspects can be gained by studying the lateral and cross-sectional morphology as well as the morphology evolution as a function of the mixing ratio of the polymers. Finally, a correlation between the morphology, photophysical processes, and device performance of the ternary and the reference binary system is achieved, explaining the differences of the power conversion efficiency between the two systems.
CRIS-Team Scopus Importer:2020-04-21
Keywords: device performance; energy-filtered transmission electron microscopy; morphology; ternary organic solar cells; transmission electron microscopy

Unraveling the influence of non-fullerene acceptor molecular packing on photovoltaic performance of organic solar cells - Ye, Linglong; Weng, Kangkang; Xu, Jinqiu; Du, Xiaoyan; Chandrabose, Sreelakshmi; Chen, Kai; Zhou, Jiadong; Han, Guangchao; Tan, Songting; Xie, Zengqi; Yi, Yuanping; Li, Ning; et al. (Nature Communications, 2020)
In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π–π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing efficient charge transport and high power conversion efficiency. Here, we demonstrate that side-chain engineering of non-fullerene acceptors could drive the fused-ring backbone assembly from a π–π stacking mode to an intermixed packing mode, and to a non-stacking mode to refine its solid-state properties. Different from the above-mentioned understanding, we find that close atom contacts in a non-stacking mode can form efficient charge transport pathway through close side atom interactions. The intermixed solid-state packing motif in active layers could enable organic solar cells with superior efficiency and reduced non-radiative recombination loss compared with devices based on molecules with the classic end-group π–π stacking mode. Our observations open a new avenue in material design that endows better photovoltaic performance.
CRIS-Team Scopus Importer:2020-12-04

Unraveling the Microstructure-Related Device Stability for Polymer Solar Cells Based on Nonfullerene Small-Molecular Acceptors - Du, Xiaoyan; Heumüller, Thomas; Gruber, Wolfgang; Almora, Osbel; Classen, Andrej; Qu, Jianfei; He, Feng; Unruh, Tobias; Li, Ning; Brabec, Christoph (Advanced Materials, 2020)
As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long-term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state-of-the-art polymer donors with a commonly used nonfullerene acceptor (NFA), IT-4F, are investigated. The active-layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion-limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene-based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA-based solar cells, which may be the key tactics to develop next-generation OSCs with high efficiency as well as excellent stability.
CRIS-Team Scopus Importer:2020-03-10
Keywords: bulk-heterojunction microstructure; morphology stability; nonfullerene acceptors; organic solar cells; photostability

Unraveling the Role of Non-Fullerene Acceptor with High Dielectric Constant in Organic Solar Cells - Zhang, Yue; He, Yakun; Zeng, Liang; Lüer, Larry; Deng, Wanyuan; Chen, Yuting; Zhou, Jiadong; Wang, Zhiqiang; Brabec, Christoph; Wu, Hongbin; Xie, Zengqi; Duan, Chunhui (Small, 2023)
Increasing the relative dielectric constant is a constant pursuit of organic semiconductors, but it often leads to multiple changes in device characteristics, hindering the establishment of a reliable relationship between dielectric constant and photovoltaic performance. Herein, a new non-fullerene acceptor named BTP-OE is reported by replacing the branched alkyl chains on Y6-BO with branched oligoethylene oxide chains. This replacement successfully increases the relative dielectric constant from 3.28 to 4.62. To surprise, BTP-OE offers consistently lower device performance relative to Y6-BO in organic solar cells (16.27% vs 17.44%) due to the losses in open-circuit voltage and fill factor. Further investigations unravel that BTP-OE has resulted in reduced electron mobility, increased trap density, enhanced first order recombination, and enlarged energetic disorder. These results demonstrate the complex relationship between dielectric constant and device performance, which provide valuable implications for the development of organic semiconductors with high dielectric constant for photovoltaic application.
CRIS-Team Scopus Importer:2023-05-26
Keywords: dielectric constant; energetic disorder; first order recombination; non-fullerene acceptors; organic solar cells

Unravelling material properties of halide perovskites by combined microwave photoconductivity and time-resolved photoluminescence spectroscopy - Kupfer, Christian; Le Corre, Vincent Marc; Li, Chaohui; Lüer, Larry; Forberich, Karen; Kato, Masashi; Osvet, Andres; Brabec, Christoph (Journal of Materials Chemistry C, 2024)
To take advantage of the huge compositional space of metal halide perovskites, a reliable and fast characterization method is required. By combining time-resolved photoluminescence (trPL) with microwave photoconductivity measurements, we are able to rapidly extract fundamental material properties of promising perovskite materials from bare films. This is achieved by incorporating a rate-equation model into a fitting algorithm based on Bayesian optimization to globally fit the experimental results. Including microwave photoconductivity measurements into our analysis significantly enhances the accuracy of estimating recombination rate constants. Additionally, it grants us the ability to determine the ratio of electron and hole mobilities, a crucial charge transport property that cannot be obtained solely through time-resolved photoluminescence measurements. Furthermore, the high sensitivity of microwave photoconductivity measurements to long-lived trap states yields information about the trap-assisted recombination, which is a major source of efficiency loss for solar devices and is difficult to assess from pure trPL measurements. In this work, we introduce the fundamental principles of microwave photoconductivity, along with the employed rate-equation model and fitting techniques. We then outline the procedure for conducting measurements and extracting the recombination rate constants, trap density, and mobility ratio. Subsequently, we highlight the benefits of combining both methods through a comparison of results from fits using only trPL with those from fits incorporating combined signals on a simulated dataset. We also discuss the merits of conducting intensity-dependent measurements. Finally, we present the annealing-dependent results obtained from a highly relevant triple-cation, mixed-halide perovskite, FA0.82MA0.13Cs0.05Pb(I0.87Br0.13)3, as an application example, where we uncover the presence of long-lived traps beyond 10 μs.
Created from Fastlane, Scopus look-up

Unveiling the Role of BODIPY Dyes as Small-Molecule Hole Transport Material in Inverted Planar Perovskite Solar Cells - Seoneray, Isabel; Wu, Jianchang; Rocha-Ortiz, Juan S.; Bornschlegl, Andreas; Barabash, Anastasia; Wang, Yunuo; Lüer, Larry; Hauch, Jens; García, Angélica; Zapata-Rivera, Jhon; Brabec, Christoph J.; Ortiz, Alejandro (Solar RRL, 2024)
Perovskite solar cells (PSCs) have become a research hotspot since their dramatic increase in power conversion efficiency (PCE), surpassing 26% due to advances in cell engineering and interfacial layers. Within the last factor, hole transporting materials play a crucial role in enhancing device performance and stability. Among several molecular building blocks, BODIPYs are attractive for the design of novel hole transporting material (HTMs) due to their outstanding photophysical and charge transport properties easily tuned by synthetic modifications. Herein, the synthesis of five new BODIPY-based HTMs PyBDP 1–5 are reported, functionalized at the meso- and α- positions with pyrenyl and arylamino units, respectively. The resulting compounds exhibit broad absorption in the visible region, remarkable thermal stability, narrow bandgaps, suitable energy levels, and good hole extraction capability, as subtracted from experimental and computational characterizations. The performance of the BODIPY derivatives as HTMs is evaluated in planar inverted (p-i-n) PSCs and compared to commonly used PTAA, resulting in highly efficient systems, reaching PCEs very close to that obtained with the reference polymer (21.51%). The incorporation of these BODIPY-based HTMs result in an outstanding PCE of 20.37% for devices including PyBDP-1 and 19.97% for devises containing PyBDP-3, thus demonstrating that BODIPY derivatives are a promising alternative to obtain simple and efficient organic HTMs.
CRIS-Team Scopus Importer:2024-05-31
Keywords: arylamine; BODIPY; hole-transporting material; perovskite solar cells; pyrene

Up-conversion semiconducting MoO 3:Yb/Er nanocomposites as buffer layer in organic solar cells - Wang, Hao; Stubhan, Tobias; Osvet, Andres; Litzov, Ivan; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2012)
The spectral sensitivity of organic solar cells can be sensitized in the near-IR and IR by incorporating Yb/Er-doped MoO up-conversion nanocomposites. Poly-3-hexyl thiophene (P3HT) and [6,6] phenyl C butyric acid methyl ester (PC BM) were chosen for the active layer. Yb/Er-doped MoO nanoparticles with dual functionality were solution processed as charge selective interface layer. On the one hand, the electrical functionality of MoO as hole extraction layer is demonstrated. On the other hand, up-conversion emission from Yb sensitized Er dopants in the MoO nanoparticles is observed. Since the photocurrent generated from up-conversion with 975 nm excitation was found to be in competition with photocurrent contributions from a charge transfer complex absorbing also in the near IR, up-conversion from a Yb/Er doped MoO layer within an organic solar cell is proven spectroscopically and demonstrated by analyzing the photocurrent response of the Yb/Er doped cell and the undoped cell to the excitation intensity. The concept to integrate up-conversion functionality into metal oxide (MOx) based charge extraction layer suggests a promising and practical pathway for improving the near IR and IR sensitivity of thin film solar cells. © 2012 Elsevier B.V.
Keywords: Interface layer; Organic solar cell; Solution-processable; Up-conversion semiconducting MoO 3

Upscaling of Perovskite Photovoltaics - Jang, Dongju; Yang, Fu; Dong, Lirong; Brabec, Christoph; Egelhaaf, Hans-Joachim (wiley, 2021)
Hybrid organic-inorganic perovskite solar cells have reached a certified efficiency of over 25% during the past decade, which has attracted significant attention as a promising candidate for photovoltaics (PVs) applications. However, the most efficient perovskite solar cells were performed by the spin coating technique, which is extremely limited in commercialization. Besides, the efficiencies of large-area perovskite cells and modules are still significantly lower. There are still some challenges that need to overcome to bridge the efficiency gap between small-area perovskite solar cells and large-area perovskite devices. The most important challenge is how to prepare high-quality perovskite layers using cheaper and scalable techniques with high reproducibility. On the other hand, select and deposit charge extraction/transport layers, and the bottom and top electrodes using cheaper and scalable techniques are also important. In this review, recent progress and challenges of the scalable technologies for solution-based coating and printing methods are summarized and analyzed based on the current progress in scalable technologies for solution-based preparation of large-area perovskite PVs. Based on our analysis, we try to point out the major challenges to overcome to successfully bridge the efficiency gap between small-area perovskite solar cells and large-area perovskite devices. Finally, strategies and opportunities are proposed to promote stable and efficient large-area perovskite PVs toward commercialization.
CRIS-Team Scopus Importer:2024-12-16
Keywords: module; perovskite solar cells; roll-to-roll production; upscaling

Upscaling Solution-Processed Perovskite Photovoltaics - Yang, Fu; Jang, Dongju; Dong, Lirong; Qiu, Shudi; Distler, Andreas; Li, Ning; Brabec, Christoph; Egelhaaf, Hans-Joachim (Advanced Energy Materials, 2021)
The performance of hybrid organic–inorganic perovskite solar cells has reached a certified efficiency of 25.5% over the past decade, which has attracted significant attention as a promising candidate for photovoltaic (PV) applications. However, the most efficient perovskite solar cells were produced by the technique of spin coating, which is extremely limited in terms of upscaling production for the commercialization of the technology. Furthermore, the efficiencies of large-area perovskite modules are still significantly lower than those of lab size solar cells. Thus, there are still some challenges that need to be overcome to bridge the efficiency gap between small-area perovskite solar cells and large-area perovskite devices. The first challenge lies in preparing high-quality perovskite layers by low-cost and scalable techniques with high reproducibility. Second, selecting and depositing charge extraction layers as well as bottom and top electrodes by scalable and low-cost techniques are essential tasks. In this review, recent progress and challenges of scalable technologies for solution-based coating and printing of perovskite PVs are summarized and analyzed. Based on the analysis, strategies and opportunities are proposed to promote the development of stable and efficient large-area perovskite PV toward commercialization.
CRIS-Team Scopus Importer:2021-10-22
Keywords: high-efficiency; long-term stability; perovskite photovoltaics; roll-to-roll production; scalable coating

Utilization of Inverter Operating Point Shifts as a Quality Assessment Tool for Photovoltaic Systems - Dalsaß, Manuel; Schmitt, Pascal; Buerhop, Claudia; Luchscheider, Philipp; Hauch, Jens; Brabec, Christoph; Camus, Christian (Ieee Photonics Journal, 2018)
This paper presents a novel approach for the automated identification of electrically impaired strings in photovoltaic (PV) systems, which is solely based on monitored inverter data. By shifting the inverter operating points of two or more strings to a common voltage level on the direct current side, the effect of defects such as bypassed substrings is actively amplified so that defects can be detected reliably. The presented Common Voltage Test is illustrated using an electrical simulation based on the extended two-diode model. The effectiveness of the proposed approach is demonstrated using real monitoring data of an inverter in a PV power plant in Bavaria, Germany. For validation, electrically defective PVmodules within the PV system are identified and located using aerial infrared thermography. Facilitating an automated reliable detection of faulty strings in PV systems, the approach is especially useful for large-scale PV power plants since only the faulty strings need to be examined in greater detail. Consequently, the maintenance becomes more efficient so that the maintenance costs may be reduced significantly.
Keywords: Fault detection; inverters; monitoring; photovoltaic (PV) systems.

Utilization of Temperature-Sweeping Capacitive Techniques to Evaluate Band Gap Defect Densities in Photovoltaic Perovskites - Almora Rodriguez, Osbel; García-Batlle, Marisé; Garcia-Belmonte, Germà (Journal of Physical Chemistry Letters, 2019)
Capacitive techniques, routinely used for solar cell parameter extraction, probe the voltage-modulation of the depletion layer capacitance isothermally as well as under varying temperature. Also defect states within the semiconductor band-gap respond to such stimuli. Although extensively used, capacitive methods have found difficulties when applied to elucidate bulk defect bands in photovoltaic perovskites. This is so because perovskite solar cells (PSCs) actually exhibit some intriguing capacitive features hardly connected to electronic defect dynamics. The commonly reported excess capacitance observed at low frequencies is originated by outer interface mechanisms and has a direct repercussion on the evaluation of band-gap defect levels. Starting by updating previous observations on Mott-Schottky (MS) analysis in PSCs, it is discussed how the thermal admittance spectroscopy (TAS) and the deep level transient spectroscopy (DLTS) characterization techniques present spectra with overlapping or even “fake” peaks caused by the mobile ion-related, interfacial excess capacitance. These capacitive techniques, when used uncritically, may be misleading and produce wrong outcomes.<br

Utilizing the unique charge extraction properties of antimony tin oxide nanoparticles for efficient and stable organic photovoltaics - Liu, Chao; Félix, Roberto; Forberich, Karen; Du, Xiaoyan; Heumüller, Thomas; Matt, Gebhard; Gu, Ening; Wortmann, Jonas; Zhao, Yicheng; Cao, Yuanyuan; He, Yakun; Ying, Lei; et al. (Nano Energy, 2021)
Simultaneously enhancing device performance and longevity, as well as balancing the requirements on cost, scalability, and simplification of processing, is the goal of interface engineering of organic solar cells (OSCs). In our work, we strategically introduce antimony (Sb3+) cations into an efficient and generic n-type SnO2 nanoparticles (NPs) host during the scalable flame spray pyrolysis synthesis. Accordingly, a significant switch of conduction property from an n-type character to a p-type character is observed, with a corresponding shift in the work function (WF) from 4.01 ± 0.02 eV for pristine SnO2 NPs to 5.28 ± 0.02 eV for SnO2 NPs with 20 mol. % Sb content (ATO). Both pristine SnO2 and ATO NPs with fine-tuned optoelectronic properties exhibit remarkable charge carrier extraction properties, excellent UV resistance and photo-stability being compatible with various state-of-the-art OSCs systems. The reliable and scalable pristine SnO2 and ATO NPs processed by doctor-blading in air demand no complex post-treatment. Our work offers a simple but unique approach to accelerate the development of advanced interfacial materials, which could circumvent the major existing interfacial problems in solution-processed OSCs.
CRIS-Team Scopus Importer:2021-08-20
Keywords: Antimony doped tin oxide; Doping mechanism; Interface engineering; Metal oxide nanoparticles; Organic photovoltaics

UV emitting single crystalline film scintillators grown by LPE method: Current status and perspective - Zorenko, Yuriy; Gorbenko, Vitaliy; Savchyn, Volodymyr; Voznyak, Taras; Batentschuk, Miroslaw; Winnacker, Albrecht; Xia, Qi; Brabec, Christoph (Materials Research Society Symposium Proceedings, 2012)
This work is dedicated to the development of new type of UV phosphors based on single crystalline films (SCF) of aluminum garnet compounds grown by liquid phase epitaxy (LPE). The development of two types of UV emitting SCF scintillators is reported in this work: 1) Pr-doped SCF of Y-Lu-Al garnet having the intensive Pr f-d luminescence in the 300-400 run spectral ranges with a decay time of about 13-18 ns; 2) SCF of Y-Lu-Al-garnet doped with Sc isoelectronic impurity emitting in the 290-400 nm range due to the formation of the Sc and Sc centers with a luminescence decay time in the order of several hundred ns. © 2011 Materials Research Society.
Keywords: Current status; Decay time; Isoelectronic impurities; Luminescence decay time; Single-crystalline film; Spectral range; UV-Emitting Engineering controlled terms: Aluminum; Crystalline materials; Films; Garnets; Ionization; Luminescence; Scintillation; Scintillation counters Engineering main heading: Phosphors

UV-Inert ZnTiO3 Electron Selective Layer for Photostable Perovskite Solar Cells - Wei, Jing; Guo, Fengwan; Liu, Bing; Sun, Xiangyu; Wang, Xi; Yang, Zijiang; Xu, Kun; Lei, Ming; Zhao, Yicheng; Xu, Dongsheng (Advanced Energy Materials, 2019)
Although planar-structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly-used TiO2 electron selective layer has a strong photocatalytic effect on perovskite/TiO2 interface when TiO2 is excited by UV light. Here a UV-inert ZnTiO3 is reported as the electron selective layer in planar PSCs. ZnTiO3 is a perovskite-structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO3/Cs(0.05)FA(0.81)MA(0.14)PbI(2.55)Br(0.45)/Sprio-MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm(-2)), compared with 55% for TiO2-based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.
CRIS-Team WoS Importer:2019-10-04

Vacuum-Free and Highly Dense Nanoparticle Based Low-Band-Gap CuInSe2 Thin-Films Manufactured by Face-to-Face Annealing with Application of Uniaxial Mechanical Pressure - Schuster, Matthias; Stapf, Dominik; Osterrieder, Tobias; Barthel, Vincent; Wellmann, Peter (Coatings, 2019)
Copper indium gallium sulfo-selenide (CIGS) based solar cells show the highest conversion eciencies among all thin-film photovoltaic competition. However, the absorber material manufacturing is in most cases dependent on vacuum-technology like sputtering and evaporation, and the use of toxic and environmentally harmful substances like H2Se. In this work, the goal to fabricate dense, coarse grained CuInSe2 (CISe) thin-films with vacuum-free processing based on nanoparticle (NP) precursors was achieved. Bimetallic copper-indium, elemental selenium and binary selenide (Cu2-xSe and In2Se3) NPs were synthesized by wet-chemical methods and dispersed in nontoxic solvents. Layer-stacks from these inks were printed on molybdenum coated float-glass-substrates via doctor-blading. During the temperature treatment, a face-to-face technique and mechanically applied pressure were used to transform the precursor-stacks into dense CuInSe2 films. By combining liquid phase sintering and pressure sintering, and using a seeding layer later on, issues like high porosity, oxidation, or selenium- and indium-depletion were overcome. There was no need for external Se atmosphere or H2Se gas, as all of the Se was directly in the precursor and could not leave the face-to-face sandwich. All thin-films were characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diraction (XRD), and UV/vis spectroscopy. Dense CISe layers with a thickness of about 2–3 m and low band gap energies of 0.93–0.97 eV were formed in this work, which show potential to be used as a solar cell absorber.<br
Keywords: low-band-gap CuInSe2; solar cell absorber; nanoparticles; vacuum-free deposition; face-to-face annealing; uniaxial pressure; liquid phase sintering; pressure sintering

Vapor Growth of 3C-SiC Using the Transition Layer of 3C-SiC on Si CVD Templates - Schuh, Philipp; Künecke, Ulrike; Litrico, G.; Mauceri, M.; La Via, F.; Monnoye, Sylvain; Zielinski, Marcin; Wellmann, Peter (Materials Science Forum, 2019)
We present three different ways of transferring 3C-SiC layers grown on silicon on top of a SiC carrier using a carbon glue layer. Our main focus was upon the growth on the transition layer, as 3C-SiC does not feature any polarities in the <100> or <-100> direction. We realized a stable and reproducible process by following a wet chemical approach, dealing with the difficulties of handling thin but freestanding 3C-SiC layers. By using this way, we transferred approximately 130 μm thick pieces using their horizontal hot-wall reactor (M10), which were chemo mechanically polished and afterwards fixed on our SiC carriers. Implementing such a seeding stack into our growth setup, we managed to grow between 20 μm and 130 μm thick layers on top. We have proven the possibility to grow on the transition layer. Furthermore, we observed a slight reduction in protrusion density, which is currently one of the main defects in such layers.
Keywords: 3C-SiC; sublimation epitaxy; transition layer; transfer; protrusions; bulk-like

Vapor growth of SiC bulk crystals and its challenge of doping - Wellmann, Peter; Müller, Ralf; Queren, Desiree; Sakwe, Aloysius; Pons, Michel (Surface & Coatings Technology, 2006)
The paper reviews the so-called Modified-PVT (M-PVT) technique which combines the state of the art PVT technique for SiC crystal growth with physical and chemical vapor deposition (PVD and CVD) for fine tuning of growth parameters and improved doping. Using this technique, currently the highest aluminum doping levels and lowest resistivity values in p-type bulk SiC were achieved that for the first time meet device fabrication needs. The paper will address fundamentals of the Modified-PVT technique including a comparison of experimental results with numerical modeling of the gas flow. As additional gas feeding helium, helium-aluminum vapor for p-type doping, phosphine for n-type doping and propane for fine tuning of the C/Si gas phase composition will be discussed. So far, the M-PVT concept, i.e. mixture of conventional PVT and fine tuning by PVD/CVD, enables the most flexible doping of SiC single crystals. (c) 2006 Elsevier B.V, All rights reserved.
Keywords: Silicon carbide; crystal growth; doping; modified physical vapor deposition (M-PVT); Al-doping; gas flow; numerical modeling

Velo-CT – A Collaborative 3D Racing Game as Incentive for Medical Image Annotation - Sukesh, Richin; Goshika, Sai Kishore; Narayanan, Srividhya Sathya; Bhombore, Vineet Vinay; Fu, Weilin; Kordon, Florian; Maier, Andreas (HEALTHINF - 14th International Conference on Health Informatics, 2021)
Introduction Recent advances in algorithmic processing of medical data are largely based on the use of deep learning algorithms that rely on large amounts of annotated data [1]. To cope with the often complex and time-consuming nature of data acquisition and annotation, data donation and distributed crowd-labeling are an appealing approach [2]. In this paper, we follow this idea and present a single/multi-player mobile game which aids in the annotation process of CT or MR scan data. Methods To provide users with an exciting and rewarding annotation experience, our game "Velo-CT" integrates labeling of 3D volume slices into a classical racing game. This is achieved by using data annotation as a key to unlock more advanced racing tracks in the game. To this end, the game has two game modes: 1) Track Racing, and 2) Image Annotation. In “Track Racing”, players compete by steering a vehicle through a racetrack. The main task is to overtake the opponents and try to remain first in position until the end of the race. Checkpoints along the track allow additional guidance if players accidentally leave the racetrack. To provide a more challenging and diverse experience, respawning pick-ups can be found at random locations along the track in the form of floating CT scanners. Such pick-up can either be a speed multiplier (multiplies car speed by 1.2) or a size enhancer (increases the car size by 1.5 to threat opponents and impede their view). The racetracks are modeled based on the segmentation contour of clinical CT data, e.g. showing the respiratory region. For slice segmentation we use a simple K-means classifier. Annotation points earned after beating a track allow the player to unlock new data for the second game mode “Image Annotation”. This is where the medical expertise of the player comes into play. Based on the current progress, the player is presented several medical images which may contain abnormalities. If an abnormality is spotted, the precise location can be marked by a finger tap on the area. The more images a player annotates, the more points can be earned to unlock a new track – and eventually a more intricate anatomy to race on. Results Our game was prototyped in Unity3D [3]. As a first anatomy, respiratory tracks based on the “Low Dose CT Grand Challenge” data were constructed. First experiences by players of different gaming background were reported for the single player setting. Users report a fast-paced racing experience with a healthy amount of challenge provided by pick-ups and competitive AI opponents. The organ-aligned layouts result in tracks similar in shape, which should be addressed in the future by adding multiple anatomical regions. Conclusion Velo-CT provides an intuitive and extendable integration of the often tedious data annotation process for 3D medical data into a rewarding gaming context. The prototype is available for free download and serves as a basis for further extensions to related game modes. References [1] Maier, A., Syben, C., Lasser, T., & Riess, C. (2019). A gentle introduction to deep learning in medical image processing. Zeitschrift für Medizinische Physik, 29(2), 86-101. [2] Servadei, L., Schmidt, R., Eidelloth, C., & Maier, A. (2017, October). Medical Monkeys: A Crowdsourcing Approach to Medical Big Data. In OTM Confederated International Conferences" On the Move to Meaningful Internet Systems" (pp. 87-97). Springer, Cham. [3] Murray, J. W. (2014). C# game programming cookbook for Unity 3D. AK Peters/CRC Press. <br
Keywords: Gamification; Medical Image Annotation; Deep Learning.

Verifying Defective PV-Modules by IR-Imaging and Controlling with Module Optimizers - Buerhop, Claudia; Fecher, Frank W.; Pickel, Tobias; Häring, Adrian; Adamski, Tim; Camus, Christian; Hauch, Jens; Brabec, Christoph (33rd European Photovoltaic Solar Energy Conference and Exhibition, 2017)
Quality control and efficient operation of roof-top, residential PV-installations is an economic and technical challenge because these PV-systems often suffer from various roof orientations and inclinations as well as partial and temporal shading. Module optimizers, which track the operating point individually for each module group, are good solutions providing also additional electrical information. The purpose of this work is verifying the identification and localization of defective PVmodules with IR-imaging and with module optimizers as well as quantifying the impact on the performance. Several PV-plants were inspected and carefully analyzed. The findings using IR-imaging can clearly be classified to substring failures due to open circuit and short circuit, bypass-diode faults, cell defect, and some undefined optimizer problems. The electric data reveal that these findings are related to certain power loss and monthly or annual yield reduction. Interesting is the temporal behavior of particular defective sites, which show short-term recovery. In summary, there is a good agreement between the occasionally performed IR-findings and the continuously monitored electrical data.
Keywords: Defects; Monitoring; Yield; IR Imaging; Thermography; Module Optimization/Optimisation

Verifying defective PV-modules by IR-imaging and controlling with module optimizers - Buerhop, Claudia; Fecher, Frank W.; Pickel, Tobias; Häring, Adrian; Adamski, Tim; Camus, Christian; Hauch, Jens; Brabec, Christoph (Progress in Photovoltaics, 2018)
Quality control and efficient operation of roof‐top residential photovoltaic (PV)‐installations is an economic and technical challenge because these PV‐systems often suffer from various roof orientations and inclinations as well as partial and temporal shading. Module optimizers, which track the operating point individually for each module group, are good solutions providing also additional electrical information. The purpose of this work is verifying the identification and localization of defective PV‐modules with infrared (IR)–imaging and with module optimizers as well as quantifying the impact on the performance. Several PV‐plants were inspected and carefully analyzed. The findings using IR‐imaging can clearly be classified to substring failures due to open circuit and short circuit, bypass‐diode faults, cell defect, and some undefined optimizer problems. The electric data reveal that these findings are related to certain power loss and monthly or annual yield reduction. Interesting is the temporal behavior of particular defective sites, which show short‐term recovery. In summary, there is a good agreement between the occasionally performed IR‐findings and the continuously monitored electrical data.
Keywords: Defects; IR-imaging; Module optimizer; Monitoring; Thermography; Yield

Vertically Aligned 2D/3D Pb-Sn Perovskites with Enhanced Charge Extraction and Suppressed Phase Segregation for Efficient Printable Solar Cells - Li, Chaohui; Pan, Yamin; Hu, Jinlong; Qiu, Shudi; Zhang, Cuiling; Yang, Yuzhao; Chen, Shi; Liu, Xianhu; Brabec, Christoph; Nazeeruddin, Mohammad Khaja; Mai, Yaohua; Guo, Fei (ACS Energy Letters, 2020)
The concept of mixed 2D/3D heterostructures has emerged as an effective method for improving the stability of lead halide perovskite solar cells, which has been, however, rarely reported in lead-tin (Pb-Sn) mixed perovskite devices. Here, we report a scalable process for depositing mixed 2D/3D Pb-Sn perovskite solar cells that deliver remarkably enhanced efficiency and stability compared to their 3D counterparts. The incorporation of a small amount (3.75%) of an organic cation 2-(4-fluorophenyl)ethylammonium iodide induces the growth of highly oriented Pb-Sn perovskite crystals perpendicularly aligned with the substrate. Moreover, for the first time, phase segregation is observed in pristine 3D Pb-Sn perovskites, which is suppressed due to the presence of the 2D perovskites. Accordingly, a high current density of 28.42 mA cm(-2) is obtained due to the markedly enhanced spectral response and charge extraction. Eventually, mixed 2D/3D Pb-Sn perovskite devices with a band gap of 1.33 eV yield efficiencies as high as 17.51% and in parallel exhibit good stability.
Created from Fastlane, WoS look-up

Visible and Near-Infrared Imaging with Nonfullerene-Based Photodetectors - Gasparini, Nicola; Gregori, Alberto; Salvador, Michael; Biele, Markus; Wadsworth, Andrew; Tedde, Sandro; Baran, Derya; Mcculloch, Iain; Brabec, Christoph (Advanced Materials Technologies, 2018)
The solution-processed organic photodetectors underpin an emerging technology with inherent implications in the biological sensors and imaging displays. Conventional organic photodiodes, the core element of an organic photodetector, rely mainly on fullerene-based acceptors, which in combination with high and middle bandgap donors, limit the spectral photosensitivity to the visible range. Even in the case of low bandgap polymers the oscillator strength and thus the extinction coefficient are usually limited in the NIR due to the nature of molecular orbital hybridization. Instead, it is showed that pairing prototypical poly(3-hexylthiophene) (P3 HT) with rhodanine-benzothiadiazolecoupled indacenodithiophene (IDTBR), a nonfullerene electron acceptor absorbing beyond 750 nm, as the photoactive material of a simple photodiode results in a highly efficient organic photodetector with a record responsivity of 0.42 A W-1 and external quantum efficiency (EQE) of 69% in the NIR (755 nm) Nonfullerene-based photodiodes are processed on amorphous silicon active matrix backplanes to realize large area flat panel photodetector imagers able to detect objects under visible and NIR light conditions with an exceptional combination of responsivity, dynamic response and image crosstalk.
Keywords: imager; large area devices; near-infrared detection; nonfullerene acceptors; organic photodetectors

Visualizing and Suppressing Nonradiative Losses in High Open-Circuit Voltage n-i-p-Type CsPbI3 Perovskite Solar Cells - Meng, Wei; Hou, Yi; Karl, André; Gu, Ening; Tang, Xiaofeng; Osvet, Andres; Zhang, Kaicheng; Zhao, Yicheng; Du, Xiaoyan; García Cerrillo, José; Li, Ning; Brabec, Christoph (ACS Energy Letters, 2020)
Cesium lead iodide (CsPbI3) has attracted increasing attention for its photovoltaic applications, owing to its thermal stability and suitable band gap for tandem solar cells. However, the severe nonradiative recombination losses in CsPbI3-based perovskite solar cells generally restrict their open-circuit voltage (VOC) to the range of 0.9 to 1.1 V. This work uniquely reports a method to visualize all defect-assisted recombination pathways with photoluminescence (PL) techniques. Visible and valuable insight into the reduction of defect densities on a micrometer scale was obtained by the bottom surface and bulk passivation with barium hydroxide and trioctylphosphine oxide. The dual effects successfully improve the VOC of the solar cell from 0.87 to 1.17 V. These results highlight the potential of hyperspectral PL imaging as a powerful tool to give guidance to further suppress the nonradiative VOC losses in all-inorganic perovskites.
CRIS-Team Scopus Importer:2020-01-21

Visualizing the performance loss of solar cells by IR thermography - an evaluation study on CIGS with artificially induced defects - Vetter, Andreas; Babbe, Finn S.; Hofbeck, Bernhard; Kubis, Peter; Richter, Michael; Heise, Stephan J.; Ohland, Joerg; Riedel, Ingo; Brabec, Christoph (Progress in Photovoltaics: Research and Applications, 2016)
Local electric defects may result in considerable performance losses in solar cells. Infrared (IR) thermography is one important tool to detect these defects on photovoltaic modules. Qualitative interpretation of IR images has been carried out successfully, but quantitative interpretation has been hampered by the lack of “calibration” defects. The aims of this study are to (i) establish methods to induce well-defined electric defects in thin-film solar cells serving as “calibration” defects and to (ii) assess the accuracy of IR imaging methods by using these artificially induced defects. This approach paves the way for improving quality control methods based on imaging in photovoltaic. We created ohmic defects (“shunts”) by using a focused ion beam and weak diodes (“interface shunts”) by applying a femto-second laser at rather low power on copper indium gallium selenide cells. The defects can be induced precisely and reproducibly, and the severity of the defects on the electrical performance can be well adjusted by focused ion beam/laser parameters. The successive assessment of the IR measurement (ILIT-Voc) revealed that this method can predict the losses in P (maximal power extractable) with a mean error of below 10%. Copyright Â© 2016 John Wiley & Sons, Ltd.
Keywords: defects; imaging; perfomance; PV modules

Volatile Perovskite Precursor Ink Enables Window Printing of Phase-Pure FAPbI3 Perovskite Solar Cells and Modules in Ambient Atmosphere - Yuan, Linhao; Chen, Xining; Guo, Xianming; Huang, Shihao; Wu, Xiaoxiao; Shen, Yunxiu; Gu, Hao; Chen, Yujin; Zeng, Guixiang; Egelhaaf, Hans-Joachim; Brabec, Christoph; Yang, Fu; et al. (Angewandte Chemie International Edition, 2024)
Despite the great success of perovskite photovoltaics in terms of device efficiency and stability using laboratory-scale spin-coating methods, the demand for high-throughput and cost-effective solutions remains unresolved and rarely reported because of the complicated nature of perovskite crystallization. In this work, we propose a stable precursor ink design strategy to control the solvent volatilization and perovskite crystallization to enable the wide speed window printing (0.3 to 18.0 m/min) of phase-pure FAPbI3 perovskite solar cells (pero-SCs) in ambient atmosphere. The FAPbI3 perovskite precursor ink uses volatile acetonitrile (ACN) as the main solvent with DMF and DMSO as coordination additives is beneficial to improve the ink stability, inhibit the coffee rings, and the complicated intermediate FAPbI3 phases, delivering high-quality pin-hole free and phase-pure FAPbI3 perovskite films with large-scale uniformity. Ultimately, small-area FAPbI3 pero-SCs (0.062 cm2) and large-area modules (15.64 cm2) achieved remarkable efficiencies of 24.32 % and 21.90 %, respectively, whereas the PCE of the devices can be maintained at 23.76 % when the printing speed increases to 18.0 m/min. Specifically, the unencapsulated device exhibits superior operational stability with T90>1350 h. This work represents a step towards the scalable, cost-effective manufacturing of perovskite photovoltaics with both high performance and high throughput.
CRIS-Team Scopus Importer:2024-02-16
Keywords: Large-Area Printing; Perovskite Solar Cells; Solar Module; Volatile Solvent; Wide Speed Window

Voltage-Dependent Photoluminescence and How It Correlates with the Fill Factor and Open-Circuit Voltage in Perovskite Solar Cells - Stolterfoht, Martin; Le Corre, Vincent Marc; Feuerstein, Markus; Caprioglio, Pietro; Koster, L Jan Anton; Neher, Dieter (ACS Energy Letters, 2019)
Optimizing the photoluminescence (PL) yield of a solar cell has long been recognized as a key principle to maximize the power conversion efficiency. While PL measurements are routinely applied to perovskite films and solar cells under open circuit conditions (V-OC), it remains unclear how the emission depends on the applied voltage. Here, we performed PL(V) measurements on perovskite cells with different hole transport layer thicknesses and doping concentrations, resulting in remarkably different fill factors (FFs). The results reveal that PL(V) mirrors the current-voltage (JV) characteristics in the power-generating regime, which highlights an interesting correlation between radiative and nonradiative recombination losses. In particular, high FF devices show a rapid quenching of PL(V) from open-circuit to the maximum power point. We conclude that, while the PL has to be maximized at V-OC at lower biases < V-OC the PL must be rapidly quenched as charges need to be extracted prior to recombination.

Washing away barriers - Li, Ning; Brabec, Christoph (Nature Energy, 2017)
Light-weight and stretchable, organic photovoltaics offer unique integration prospects. Now, organic solar cells and modules can also be washed while maintaining good photoconversion efficiencies. © 2017 The Publisher

Water ingress in encapsulated inverted organic solar cells: Correlating infrared imaging and photovoltaic performance - Adams, Jens; Salvador, Michael Filipe; Lucera, Luca; Langner, Stefan; Spyropoulos, George D.; Fecher, Frank; Voigt, Monika; Dowland, Simon A.; Osvet, Andres; Egelhaaf, Hans-Joachim; Brabec, Christoph (Advanced Energy Materials, 2015)
Understanding the degradation and failure mechanisms of organic photovoltaic devices is a key requirement for this technology to mature toward a reliable product. Here, an investigation on accelerated temperature and moisture long-term stability testing (>20 000 h) of inverted and glass-encapsulated poly(3-hexylthiophene)/phenyl-C-butyric acid methyl ester solar cells is presented. The degradation kinetics is analyzed using the Arrhenius model and the resulting activation energy for the diffusion of water is measured to be ≈43 kJ mol. Through comparison of electroluminescence imaging, lock-in thermography, and photoluminescence mapping, the device performance is correlated with the loss of effective cell area and it is shown that the reaction of water at the hole extraction/active layer interface is likely to be the dominant cause for long-term device failure. The diffusion of water through the packaged solar cell is described using classical diffusion theory. Based on an analytical solution of a simple diffusion model, the diffusion coefficient is estimated to be 4 × 10 m s. A shelf life of 100 000 h is anticipated at 65 °C/85% RH using a 9.3 cm wide protective adhesive rim. The findings of this study may inform strategies for predicting lifetimes of organic solar cells and modules based on local in situ tracking of moisture-induced device performance loss using IR imaging. Lifetime limitations of current-generation inverted and encapsulated organic solar cells under damp heat conditions are identified by correlating photovoltaic device performance with lock-in IR imaging techniques. The combination of electroluminescence imaging and water diffusion modeling may inform strategies for predicting early performance losses in solar cells and modules.
Keywords: electroluminescence imaging; lock-in thermography; long-term stability; organic photovoltaics; water diffusion

Wet leakage resistance development of modules with various backsheet types - Buerhop, Claudia; Stroyuk, Oleksandr; Zoecklein, Julian; Pickel, Tobias; Hauch, Jens; Peters, Ian Marius (Progress in Photovoltaics: Research and Applications, 2021)
Over the last years, a significant number of inverter shutdowns most likely due to degraded backsheets have been reported by operators, investors, and manufacturers. We investigated water ingress into different backsheets, and the resulting risk for inverter shutdowns. For studying pending insulation issues of inverters, we analyzed exemplarily a 5-MWp photovoltaic (PV) power station with 20,530 PV modules and 314 inverters. For backsheet identification on-site near-infrared absorption spectroscopic measurements of 518 PV modules from 20 inverters were carried out. In the lab, wet leakage tests provide kinetic data of insulation resistances for different backsheet materials. Historic ground impedance data logged by the inverter were evaluated to show the temporal evolution of inverters with different backsheets. We observe different kinetics and drops in leakage resistance for different backsheet types based on polyamide (PA) and air-side fluorinated coating (FC), and distinguish between two types of behavior. “PA-like” is characterized by moderate leakage resistance loss and slower performance reduction with regard to insulation in the field. “FC-like” is characterized by high susceptibility to water and a high leakage resistance loss rate for PV modules after 6–8 years of operation. These characteristics are indicated when measuring inverter GI in dependence of temperature and humidity. For PA-based BSs, the reduction in ground impedance (GI) is steady over 8 years. We observe incidences with GI below the critical value of 400 kΩ on 2.8% of all days. For FC-based BSs, we also observe steady decrease of average GI initially, yet after 6 years in the field, we see a sudden and rapid decline for 20% of those inverters. GI falling below the critical value is observed on up to 5% of all days, with the trend increasing. Therefore we, expect this problem to become even more severe for longer operating times.
CRIS-Team Scopus Importer:2021-10-22
Keywords: backsheet; ground impedance; leakage current; water susceptibility

Wide Bandgap Semiconductors for Power Electronics: Materials, Devices, Applications - Wellmann, Peter; Ohtani, Noboru; Rupp, Roland (wiley, 2021)
Wide Bandgap Semiconductors for Power Electronic A guide to the field of wide bandgap semiconductor technology Wide Bandgap Semiconductors for Power Electronics is a comprehensive and authoritative guide to wide bandgap materials silicon carbide, gallium nitride, diamond and gallium(III) oxide. With contributions from an international panel of experts, the book offers detailed coverage of the growth of these materials, their characterization, and how they are used in a variety of power electronics devices such as transistors and diodes and in the areas of quantum information and hybrid electric vehicles. The book is filled with the most recent developments in the burgeoning field of wide bandgap semiconductor technology and includes information from cutting-edge semiconductor companies as well as material from leading universities and research institutions. By taking both scholarly and industrial perspectives, the book is designed to be a useful resource for scientists, academics, and corporate researchers and developers. This important book: Presents a review of wide bandgap materials and recent developments Links the high potential of wide bandgap semiconductors with the technological implementation capabilities Offers a unique combination of academic and industrial perspectives Meets the demand for a resource that addresses wide bandgap materials in a comprehensive manner Written for materials scientists, semiconductor physicists, electrical engineers, Wide Bandgap Semiconductors for Power Electronics provides a state of the art guide to the technology and application of SiC and related wide bandgap materials.
CRIS-Team Scopus Importer:2024-12-16

Wirtschaftlichkeitsbetrachtung von Haushaltsspeichern und realdatengestützte Untersuchung des elektrischen Eigenverbrauchs von PV-Strom - Stegner, Christoph; Bogenrieder, Josef; Auer, Sebastian; Luchscheider, Philipp; German, Reinhard; Brabec, Christoph (Nachhaltige Energieversorgung und Integration von Speichern: Tagungsband zur NEIS 2015, 2015)
UnivIS-Import:2016-06-01:Pub.2015.tech.IMMD.IMMD7.wirtsc

X-Ray Computed Tomography specifically adapted for monitoring of dynamic processes in material science and chemistry - Salamon, Michael; Reisinger, S.; Schlechter, T.; Schmitt, M.; Hofmann, T.; Uhlmann, Norman; Schug, Sebastian; Arlt, Wolfgang; Fahlbusch, Lars; Arzig, Matthias; Wellmann, Peter (WCIPT8 - 8th WORLD CONGRESS ON INDUSTRIAL PROCESS TOMOGRAPHY, 2016)
X-ray computed tomography (CT) is an established method in non-destructive testing / evaluation (NDT / NDE) applications. A wide range of CT systems is available to the market distinct mainly by resolution properties. The process X-ray tomography application requires for an adaption of the process device which in some cases can be mounted on the rotation stage of the standard CT apparatus. Most representative examples are compact fatigue testing devices providing multiaxial forces to the tensile specimen encapsulated in a mostly X-ray transparent housing. This procedure is usually applied to relatively small and isolated samples. For processes that cannot be measured in an isolated environment, due to their size or other boundaries application specific CT systems can be developed using a large variety of X-ray components available on the market. In this contribution the Fraunhofer Development Centre for X-ray Technology (EZRT) shows application specific solutions realized in the past years for process monitoring in material science and chemistry. One focus of the contribution is the CT of packed distillation columns applied for the investigation of the fluid dynamics and transportation processes in different packing systems at the Institute of Separation Science and Technology, University of Erlangen-Nürnberg. Fluid dynamic parameters in distillation columns (like interfacial area, liquid hold-up and liquid film thickness on the packing or between two liquid phases) can be directly estimated from the recorded CT images of a packing cross section at different height positions along the column in operation. A segmentation process allows for a quantitative evaluation of the fluid behaviour corresponding to the packaging. The CT system is adapted to scan a chromatography or distillation column in its vertical orientation using a horizontally rotating X-ray gantry setup allowing for gravity affected measurements relevant in e.g. low pressure fluid systems. The setup allows for versatile applications with a CT scan rate of 1 Hz at a maximum resolution down to 70 microns for objects between 50 and 200 mm. This technique replaces today the commonly applied medical CT for such kind of experiments. Another presented example for a process adapted CT setup provides an insight into the Physical Vapor Transport (PVT) process at 2200°C allowing in-situ observation of the growth of SiC bulk crystals. The actual results achieved at the Crystal Growth Lab, University of Erlangen-Nürnberg by adaption of an X-ray tomography setup to a customized growth chamber setup will be presented and an outlook on the development tasks will be given.
Keywords: X-ray tomography; 4D-CT; Fluid dynamics; Crystal growth

X-ray Diffraction, Micro-Raman and X-ray Photoemission Spectroscopic Investigations for Hydrothermally Obtained Hybrid Compounds of Delafossite CuGaO2 and Wurtzite ZnO - Choi, Minuk; Brabec, Christoph; Hayakawa, Tomokatsu (Ceramics, 2022)
P-type delafossite CuGaO2 is a wide-bandgap semiconductor for optoelectronic applications, and its lattice parameters are very similar to those of n-type semiconductor wurtzite ZnO. Accordingly, the investigation of crystalline heterostructures of CuGaO2 and ZnO has attracted significant attention. In this study, interfacial CuGaO2/ZnO hetero-compounds were examined through X-ray diffraction (XRD) analysis, confocal micro-Raman spectroscopy, and X-ray photo-electron spectroscopy (XPS). XRD and Raman analysis revealed that the hydrothermal deposition of ZnO on hexagonal platelet CuGaO2 base crystals was successful, and the subsequent reduction process could induce a unique, unprecedented reaction between CuGaO2 and ZnO, depending on the deposition parameters. XPS allowed the comparison of the binding energies (peak position and width) of the core level electrons of the constituents (Cu, Ga, Zn, and O) of the pristine CuGaO2 single crystallites and interfacial CuGaO2/ZnO hybrids. The presences of Cu2+ ions and strained GaO6 octahedra were the main characteristics of the CuGaO2/ZnO hybrid interface. The XPS and modified Auger parameter analysis gave an insight into a specific polarization of the interface, promising for further development of CuGaO2/ZnO hybrids.
CRIS-Team WoS Importer:2023-01-13

X-ray imaging: Perovskites target X-ray detection - Heiß, Wolfgang; Brabec, Christoph (Nature Photonics, 2016)
Single crystals of perovskites are currently of interest to help fathom fundamental physical parameters limiting the performance of perovskite-based polycrystalline solar cells. Now, such perovskites offer a technology platform for optoelectronic devices, such as cheap and sensitive X-ray detectors.
Keywords: Sensors and biosensors; X-rays

X-ray imaging with scintillator-sensitized hybrid organic photodetectors - Buechele, Patric; Richter, Moses; Tedde, Sandro F.; Matt, Gebhard; Ankah, Genesis N.; Fischer, Rene; Biele, Markus; Metzger, Wilhelm; Lilliu, Samuele; Bikondoa, Oier; Macdonald, J. Emyr; Brabec, Christoph; et al. (Nature Photonics, 2015)
Medical X-ray imaging requires cost-effective and high-resolution flat-panel detectors for the energy range between 20 and 120€...keV. Solution-processed photodetectors provide the opportunity to fabricate detectors with a large active area at low cost. Here, we present a disruptive approach that improves the resolution of such detectors by incorporating terbium-doped gadolinium oxysulfide scintillator particles into an organic photodetector matrix. The X-ray induced light emission from the scintillators is absorbed within hundreds of nanometres, which is negligible compared with the pixel size. Hence, optical crosstalk, a limiting factor in the resolution of scintillator-based X-ray detectors, is minimized. The concept is validated with a 256 × 256 pixel detector with a resolution of 4.75€...lp mm '1 at a MTF = 0.2, significantly better than previous stacked scintillator-based flat-panel detectors. We achieved a resolution that proves the feasibility of solution-based detectors in medical applications. Time-resolved electrical characterization showed enhanced charge carrier mobility with increased scintillator filling, which is explained by morphological changes.
Keywords: Engineering controlled terms: Cost effectiveness; Medical applications; Medical imaging; Photodetectors; Photons; Pixels; X ray analysis Electrical characterization; Flat panel detectors; Gadolinium oxysulfides; Medical X-ray imaging; Morphological changes; Optical cross-talk; Organic photodetector; Solution-processed Engineering main heading: Scintillation counters

Zellrisse - nur halb so schlimm? - Buerhop-Lutz, Claudia; Pickel, Tobias; Bemm, Andreas; Vodermayer, Christian; Glück, Bernhard; Huber, Alexander; Camus, Christian; Hauch, Jens; Brabec, Christoph (pv magazine Deutschland, 2017)
Das Wichtigste in Kürze Ein Autorenteam hat mit Zellrissen behaftete Module ein Jahr lang in einer Outdooranlage untersucht. Die Performance dieser Module war trotz Stürmen und Schnee nicht geringer als die einer Kontrollgruppe. Die Elektrolumineszenzaufnahmen zeigen, dass nach einem Jahr weder die Anzahl der Zellrisse erhöht war noch ihre Ausprägung zugenommen hatte. Die Autoren schließen daraus, dass vorhandene Zellrisse sich nicht notwendigerweise negativ auf das Degradationsverhalten auswirken und sie daher für den Betrieb weniger schlimm sind als landläufig angenommen. Ein signifikanter Einfluss auf die Stabilität der Module unter Realbedingungen konnte im Rahmen des Versuchs nicht festgestellt werden.

Zwitterionic Ionic Liquid Confer Defect Tolerance, High Conductivity, and Hydrophobicity toward Efficient Perovskite Solar Cells Exceeding 22% Efficiency - Yang, Liqun; Ma, Xiaohui; Shang, Xueni; Gao, Deyu; Wang, Chen; Li, Mengjia; Chen, Cong; Zhang, Boxue; Xu, Sai; Zheng, Shijian; Song, Hongwei (Solar RRL, 2021)
Defect passivation has been a promising route for enhancing the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, defect inhibition in two-step solution-processed (FAPbI3)1−x(MAPbBr3)x films via a rationally designed zwitterionic ionic liquid (ZIL) with 4-fluoro-phenylammonium (4FB+) as cations and tetrafluoroborate (BF4−) as anions is demonstrated. First, 4FB+ and BF4− can effectively fill vacancy defects caused by the migrated organic A-site cation and halogen anion, confirmed by X-ray photoelectron spectroscopy. Second, the 4FB+ with π conjugated benzene ring can donate electrons for carrier extraction, whereas its fluorination of the phenyl ammonium could enhance moisture blocking through the molecular packing of C-F bond. The electrical characterization, including space charge limited current and Mott–Schottky measurement, proves the enhanced carrier extraction and photovoltaic performance. Third, the pseudohalogen anion BF4− with high ionic conductivity could significantly enhance the carrier lifetime and reduce the VOC loss. As a result, the ZIL-modified PSCs can achieve a high PCE of 22.5% with excellent long-term stability maintaining more than 80% of the initial efficiency after storing in an ambient condition for 2000 h. Herein, a new paradigm toward accelerating the development of efficient and stable PSCs is opened up.
CRIS-Team Scopus Importer:2021-07-30
Keywords: 4-fluoro-phenylammonium tetrafluoroborate; defects passivation; hydrophobicity; ionic liquids; zwitterionis

Adhesion strength of ductile thin film determined by cross-sectional nanoindentation - Zhao, Dawei; Letz, Sebastian; Jank, Michael; März, Martin (International Journal of Mechanical Sciences, 2024)
Cross-sectional nanoindentation (CSN) is a powerful tool for measuring the adhesion strength of thin films. However, a quantitative assessment of interfacial properties for metal thin films on brittle substrates remains a challenge due to thin film ductility. In this work, we present a hierarchical analysis method to account for plastic dissipation during delamination. According to our observations, the CSN-induced wedge geometry is more complex than what is currently presented in literature. Therefore, we develop a three-dimensional finite element (FE) modelling strategy to simulate CSN-induced delamination. Furthermore, we present the mode mixity distribution during CSN measurement based on our simulation results. We find that a purely experimental assessment of adhesion strength by CSN for ductile thin films is imprecise due to the adhesion strength-dependent stress-state in this characterization technique. By using our combined experimental and numerical approach, the adhesion strength of EBPVD-deposited Al thin film on SiO2/Si substrate is determined as 0.577 J/m2 and an additional 20 nm thick Ti adhesion layer enhances its adhesion strength to 16.4 J/m2.
CRIS-Team Scopus Importer:2024-03-01
Keywords: Adhesion strength characterization; Cohesive zone model; Cross-sectional nanoindentation; Ductile thin film; Finite element analysis; Mode mixity

A Framework for Data-driven Explainability in Mathematical Optimization - Aigner, Kevin-Martin; Goerigk, Marc; Hartisch, Michael; Liers, Frauke; Miehlich, Arthur (The 38th Annual AAAI Conference on Artificial Intelligence, 2024)
Advancements in mathematical programming have made it possible to efficiently tackle large-scale real-world problems that were deemed intractable just a few decades ago. However, provably optimal solutions may not be accepted due to the perception of optimization software as a black box. Although well understood by scientists, this lacks easy accessibility for practitioners. Hence, we advocate for introducing the explainability of a solution as another evaluation criterion, next to its objective value, which enables us to find trade-off solutions between these two criteria. Explainability is attained by comparing against (not necessarily optimal) solutions that were implemented in similar situations in the past. Thus, solutions are preferred that exhibit similar features. Although we prove that already in simple cases the explainable model is NP-hard, we characterize relevant polynomially solvable cases such as the explainable shortest-path problem. Our numerical experiments on both artificial as well as real-world road networks show the resulting Pareto front. It turns out that the cost of enforcing explainability can be very small.<br
Keywords: explainability; mathematical optimization; data-driven optimization; bicriteria optimization

A Gold Needle Tip Array Ultrafast Electron Source with High Beam Quality - Brückner, Leon; Nauk, Constantin; Dienstbier, Philip; Gerner, Constanze; Löhrl, Bastian; Paschen, Timo; Hommelhoff, Peter (Nano Letters, 2024)
Electron sources are crucial elements in diverse applications such as electron microscopes, synchrotrons, and free-electron lasers. Nanometer-sharp needle tips are electron emitters with the highest beam quality, yet for a single needle the current is limited. Combining the emission of multiple needles promises large current yields while preserving the individual emitters’ favorable properties. We present an ultrafast electron source consisting of a lithographically fabricated array of sharp gold tips illuminated with 25 fs laser pulses. The source provides up to 2000 electrons per pulse for moderate laser peak intensities of 1011 W/cm2 and a narrow energy width of 0.5 ± 0.05 eV at low current. The electron beam has a well-behaved Gaussian profile and is highly collimated, yielding a small normalized emittance on the order of nm·rad. These properties are well suited for applications requiring both high current and spatial resolution, such as free-electron light sources and chip-based particle accelerators.<br
Keywords: nano-optics; ultrafast electron emission; field enhancement; lasers; electron sources

A Gradient-Based Method for Joint Chance-Constrained Optimization with Continuous Distributions - Bernhard, Daniela; Liers, Frauke; Stingl, Michael (2025)
The input parameters of an optimization problem are often affected by uncertainties. Chance constraints are a common way to model stochastic uncertainties in the constraints. Typically, algorithms for solving chance-constrained problems require convex functions or discrete probability distributions. In this work, we go one step further and allow non-convexities as well as continuous distributions. We propose a gradient-based approach to approximately solve joint chance-constrained models. We approximate the original problem by smoothing indicator functions. Then, the smoothed chance constraints are relaxed by penalizing their violation in the objective function. The approximation problem is solved with the Continuous Stochastic Gradient method that is an enhanced version of the stochastic gradient descent and has recently been introduced in the literature. We present a convergence theory for the smoothing and penalty approximations. Under very mild assumptions, our approach is applicable to a wide range of chance-constrained optimization problems. As an example, we illustrate its computational efficiency on difficult practical problems arising in the operation of gas networks. The numerical experiments demonstrate that the approach quickly finds nearly feasible solutions for joint chance-constrained problems with non-convex constraint functions and continuous distributions, even for realistically-sized instances.<br
https://cris.fau.de/converis/publicweb/Publication/318680259

A Material View on Extrinsic Magnetic Domain Wall Pinning in Cylindrical CoNi Nanowires - Schöbitz, Michael; Novotný, Ondrej; Trapp, Beatrix; Bochmann, Sebastian; Cagnon, Laurent; Thirion, Christophe; Massebœuf, Aurélien; Mossang, Eric; Fruchart, Olivier; Bachmann, Julien (Journal of Physical Chemistry C, 2023)
Speed and reliability of magnetic domain wall (DW) motion are key parameters that must be controlled to realize the full potential of DW-based magnetic devices for logic and memory applications. A major hindrance to this is extrinsic DW pinning at specific sites related to shape and material defects, which may be present even if the sample synthesis is well controlled. Understanding the origin of DW pinning and reducing it are especially desirable in electrochemically deposited cylindrical magnetic nanowires (NWs), for which measurements of the fascinating physics predicted by theoretical computation have been inhibited by significant pinning. We experimentally investigate DW pinning in CoxNi100-x NWs by applying quasi-static magnetic fields. Wire compositions were varied with x = 20, 30, and 40, while the microstructure was changed by annealing or by varying the pH of the electrolyte for deposition. We conclude that pinning due to grain boundaries is the dominant mechanism, decreasing inversely with both the spontaneous magnetization and grain size. Second-order effects include inhomogeneities in lattice strain and the residual magnetocrystalline anisotropy. Surface roughness, dislocations, and impurities are not expected to play a significant role in DW pinning in these wire samples.
CRIS-Team Scopus Importer:2023-02-24

Anisotropic molecular diffusion in confinement II: A model for structurally complex particles applied to transport in thin ionic liquid films - Höllring, Kevin; Baer, Andreas; Vučemilović-Alagić, Nataša; Smith, David M.; Smith, Ana-Suncana (Journal of Colloid and Interface Science, 2024)
Hypothesis: Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue and for objects of nanometric size, which structurally fluctuate on a similar time scale as they diffuse, no methodology has been established so far. We hypothesise that the complex, coupled dynamics can be captured and analysed by using a model built on the 2-dimensional Smoluchowski equation and systematic coarse-graining. Methods and simulations: For large, flexible species, a universal approach is offered that does not make any assumptions about the separation of time scales between translation and other degrees of freedom. The method is validated on Molecular Dynamics simulations of bulk systems of a family of ionic liquids with increasing cation sizes where internal degrees of freedom have little to major effects. Findings: After validation on bulk liquids, where we provide an interpretation of two diffusion constants for each species found experimentally, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Spatial variations in the diffusivities relate to interface-induced structuring of the ionic liquids. Notably, the length scales in strongly confined ionic liquids vary consistently but differently at the solid–liquid and liquid–vapour interfaces.
CRIS-Team Scopus Importer:2023-12-15
Keywords: Anisotropic diffusion; Diffusion at interfaces; Diffusion in films; Ionic liquids; Molecular liquids; Transport coefficient

Approach For Optimized Dismantling Strategies And Design For Recycling Of Electric Traction Drives - Hahn, Roman; Wieprecht, Nico; Ihne, Thorsten; Kühl, Alexander; Franke, Jörg (6th Conference on Production Systems and Logistics, 2024)
With the widespread adoption of electric vehicles (EVs), there arises a pressing need for sustainable processing of electric traction drives (ETDs) at the end of their life cycles. Current methods for dismantling and recycling ETDs often show inefficiencies in material recovery causing economic disadvantages and increased environmental impact. Of particular relevance is the recycling of rare earth (RE) magnets, which are extensively used in ETDs because of their superior magnetic properties. Given the scarcity of rare earth elements (REEs) and the environmental impact associated with their extraction, the need for efficient RE magnet recycling is of key importance. However, there is currently a lack of efficient dismantling processes and recyclability is often inadequate. By developing an optimized approach that focuses on adapted dismantling strategies and design for recycling, a critical gap in the sustainable production and disposal of electric vehicles will be closed. A crucial aspect is the design for recycling, enabling efficient recovery and reuse of materials and components. The approach presented in this paper combines different aspects such as material composition, modularity of components, standardization and good separability to optimize the design for recycling of ETDs. By providing optimized dismantling strategies as well as guidelines for the design for recycling, the transition to more environmentally sustainable transport systems is supported in addition to increased economic efficiency. <br
Keywords: Recycling; Reuse; Optimized Dismantling; Design for Recycling; Electric Traction Drives; Rare Earth Magnets; Permanent Magnet Synchronous Machine

Assessment of Hydrophilicity/Hydrophobicity in Mesoporous Silica by Combining Adsorption, Liquid Intrusion, and Solid-State NMR Spectroscopy - Collados, Carlos Cuadrado; Huber, Christoph; Söllner, Jakob; Grass, Jan-Paul; Inayat, Alexandra; Durdyyev, Rustam; Smith, Ana-Suncana; Wisser, Dorothea; Hartmann, Martin; Thommes, Matthias (Langmuir, 2024)
We have developed a comprehensive strategy for quantitatively assessing the hydrophilicity/hydrophobicity of nanoporous materials by combining advanced adsorption studies, novel liquid intrusion techniques, and solid-state NMR spectroscopy. For this, we have chosen a well-defined system of model materials, i.e., the highly ordered mesoporous silica molecular sieve SBA-15 in its pristine state and functionalized with different amounts of trimethylsilyl (TMS) groups, allowing one to accurately tailor the surface chemistry while maintaining the well-defined pore structure. For an absolute quantification of the trimethylsilyl group density, quantitative 1H solid-state NMR spectroscopy under magic angle spinning was employed. A full textural characterization of the materials was obtained by high-resolution argon 87 K adsorption, coupled with the application of dedicated methods based on nonlocal-density functional theory (NLDFT). Based on the known texture of the model materials, we developed a novel methodology allowing one to determine the effective contact angle of water adsorbed on the pore surfaces from complete wetting to nonwetting, constituting a powerful parameter for the characterization of the surface chemistry inside porous materials. The surface chemistry was found to vary from hydrophilic to hydrophobic as the TMS functionalization content was increased. For wetting and partially wetting surfaces, pore condensation of water is observed at pressures P smaller than the bulk saturation pressure p0 (i.e., at p/p0 < 1) and the effective contact angle of water on the pore walls could be derived from the water sorption isotherms. However, for nonwetting surfaces, pore condensation occurs at pressures above the saturation pressure (i.e., at p/p0 > 1). In this case, we investigated the pore filling of water (i.e., the vapor-liquid phase transition) by the application of a novel, liquid water intrusion/extrusion methodology, allowing one to derive the effective contact angle of water on the pore walls even in the case of nonwetting. Complementary molecular simulations provide density profiles of water on pristine and TMS-grafted silica surfaces (mimicking the tailored, functionalized experimental silica surfaces), which allow for a molecular view on the water adsorbate structure. Summarizing, we present a comprehensive and reliable methodology for quantitatively assessing the hydrophilicity/hydrophobicity of siliceous nanoporous materials, which has the potential to optimize applications in heterogeneous catalysis and separation (e.g., chromatography).
CRIS-Team Scopus Importer:2024-06-21

Atmospheric Plasma Spraying for Copper Coating of Ceramic Solid Electrolytes for Anode-Free Solid-State Batteries with Increased Interfacial Contact - Ockel, Manuela; Borchers, Andre; Fröhlich, Jan; Petersen, Matthias; Paschen, Timo; Christiansen, Silke; Franke, Jörg (2024 1st International Conference on Production Technologies and Systems for E-Mobility (EPTS), 2024)
Advances in solid-state battery technology are crucial for future energy storage solutions. Atmospheric plasma spraying (APS) offers a novel approach for producing copper coatings on ceramic solid electrolytes (SE) for anode-free solid-state batteries (AFSSBs), which generally promise increased safety and energy density. This study investigates the feasibility of APS for the fabrication of Cu-based current collectors with enhanced interfacial contact at the SE and high material deposition rates in comparison to physical or chemical vapor deposition of Cu. The qualitative results suggest that APS can produce Cu coatings with high interfacial contact without inducing high pressures through calendaring, which can lead to cracks in brittle solid electrolytes. This research addresses critical challenges in the production of AFSSBs with APS and evaluates the importance of tailored interface properties for optimized contact areas. The pressure induced by the accelerated particles is substantially lower compared to conventional calendaring tools. Surface characteristics of SE and coating were analyzed via laser scanning. SEM analysis showed several defects during splat formation, which reduces interfacial contact.<br

Atomic diffusion in liquid gallium and gallium-nickel alloys probed by quasielastic neutron scattering and molecular dynamic simulations - Shahzad, Anjum; Yang, Fan; Steffen, Julien; Neiß, Christian; Panchenko, Artem; Götz, Klaus; Vogel, Carola; Weißer, Matthias; Embs, Jan Peter; Petry, Winfried; Lohstroh, Wiebke; Görling, Andreas; et al. (Journal of Physics: Condensed Matter, 2024)
The atomic mobility in liquid pure gallium and a gallium-nickel alloy with 2 at% of nickel is studied experimentally by incoherent quasielastic neutron scattering. The integral diffusion coefficients for all-atom diffusion are derived from the experimental data at different temperatures. DFT-based ab-initio molecular dynamics (MD) is used to find numerically the diffusion coefficient of liquid gallium at different temperatures, and numerical theory results well agree with the experimental findings at temperatures below 500 K. Machine learning force fields derived from ab-initio molecular dynamics (AIMD) overestimate within a small 6% error the diffusion coefficient of pure gallium within the genuine AIMD. However, they better agree with experiment for pure gallium and enable the numerical finding of the diffusion coefficient of nickel in the considered melted alloy along with the diffusion coefficient of gallium and integral diffusion coefficient, that agrees with the corresponding experimental values within the error bars. The temperature dependence of the gallium diffusion coefficient D Ga ( T ) follows the Arrhenius law experimentally for all studied temperatures and below 500 K also in the numerical simulations. However, D Ga ( T ) can be well described alternatively by an Einstein-Stokes dependence with the metallic liquid viscosity following the Arrhenius law, especially for the MD simulation results at all studied temperatures. Moreover, a novel variant of the excess entropy scaling theory rationalized our findings for gallium diffusion. Obtained values of the Arrhenius activation energies are profoundly different in the competing theoretical descriptions, which is explained by different temperature-dependent prefactors in the corresponding theories. The diffusion coefficient of gallium is significantly reduced (at the same temperature) in a melted alloy with natural nickel, even at a tiny 2 at% concentration of nickel, as compared with its pure gallium value. This highly surprising behavior contradicts the existing excess entropy scaling theories and opens a venue for further research.
CRIS-Team Scopus Importer:2024-02-16
Keywords: ab-initio molecular dynamics; DFT-based machine learning force fields; gallium-nickel alloy; liquid gallium; quasielastic neutron scattering; self-diffusion

Atomic Layer Deposition of HfS2 on Functionalized Self-Assembled Monolayers on Ordered Oxide Surfaces: A Model Study under UHV Conditions - Fickenscher, Georg; Steffen, Julien; Görling, Andreas; Libuda, Jörg (Journal of Physical Chemistry C, 2024)
We have investigated the atomic layer deposition (ALD) of HfS2 on both functionalized and non-functionalized self-assembled monolayers (SAMs) grown on an atomically defined Co3O4(111) surface under ultrahigh-vacuum (UHV) conditions. In particular, we compared the growth on the hydrocarbon-terminated biphenyl-4-carboxylic acid (BP4CA, H-Ph-Ph-COOH) SAM to the OH-terminated 4′-hydroxy-4-biphenylcarboxylic acid (4HBP4CA, HO-Ph-Ph-COOH) SAM. The ALD procedure involved sequential dosing of tetrakis(dimethylamido)hafnium (TDMAH) and D2S separated by evacuation periods in between. The surface species were monitored in-situ by infrared reflection absorption spectroscopy, and the vibrational data was interpreted based on simulated spectra by density-functional theory. We observed that the deposition of HfS2 can be very efficiently controlled by the surface termination of the SAM. At 400 K, we observe no deposition at all on the hydrocarbon-terminated BP4CA SAM, while HfS2 efficiently nucleates and grows on the OH-terminated 4HBP4CA SAM. Nucleation of HfS2 is efficiently suppressed on the BP4CA SAM up to temperatures of 400 K. Above this temperature, we observe the onset of slow deposition of HfS2 also on the BP4CA SAM, suggesting that the surface coverage of the SAM decreases and the blocking effect becomes incomplete. For the functionalized 4HBP4CA SAM, we observed that the initial nucleation process comprises several growth regimes. In specific, we propose that the stoichiometry of the Hf(NMe2)nORm nuclei changes during the first ALD half cycle. Moreover, the IR spectroscopic data indicates that the Hf-containing nuclei have a major effect on the structure of the underlying SAM. Our findings suggest that the density of functional groups and the rigidity of the SAM are critical parameters which will have a major impact on the properties of the materials deposited by ALD.
CRIS-Team Scopus Importer:2024-02-09

Atomistic picture of electronic metal support interaction and the role of water - Fusek, Lukáš; Farnesi Camellone, Matteo; Ronovský, Michal; Kastenmeier, Maximilian; Skála, Tomáš; Kumar Samal, Pankaj; Tsud, Nataliya; Mehl, Sascha; Škvára, Jan; Dolák, Tomáš; Uvarov, Vitalii; Setvín, Martin; et al. (Journal of Materials Chemistry A, 2024)
Single atom catalysis (SAC) represents an emerging area of heterogeneous catalysis but faces challenges related to the low density of active sites and poor thermal stability. In this work, we present new fundamental insights into the nature of electronic metal support interactions (EMSI) coupled with cation exchange, which yield high density of atomically dispersed noble metals on defect-free terraces of cation-terminated reducible oxides. On well-ordered Co3O4(111) films, the mechanism involves temperature-controlled substitution of surface Co2+ and sub-surface Co3+ cations by Pt2+ and Pt4+ species, respectively. The cation exchange with Co2+ is coupled with the partial reduction of Co3O4(111), while the cation exchange with Co3+ involves the charge disproportionation within the Pt species. In the presence of co-adsorbed water, the Pt4+ species are stabilized at the surface in the form of triaqua complexes.
CRIS-Team Scopus Importer:2024-02-09

Bi-directional DC Charging Stations for EVs on renewable-powered LVDC Grids: Design, Sizing, Control and Testing - Gutwald, Benjamin; Lehmann, Raphael; Barth, Martin; Reichenstein, Tobias; Franke, Jörg (2024 1st International Conference on Production Technologies and Systems for E-Mobility, EPTS 2024, 2024)
E-mobility has undergone a remarkable transformation in recent years. It has progressed from the early days of a few electric vehicles (EVs) to widespread acceptance and integration into society. In the effort to address climate change, it has also become a necessity. The advancement in battery technology has enabled these EV not only to have better performance and longer ranges but also the capability to function as energy storage units for both households and businesses. This feature can prove valuable in industrial fleets, contributing substantially to grid stability and financial savings through temporary renewable energy storage and peak load balancing. DC grids provide the most flexible, efficient, and environmentally friendly charging architecture. Ideally, they are supplied directly from renewable energy generators. While bidirectional charging station prototypes for AC networks are emerging, solutions for future DC grids are still lacking.This publication evaluates the potential of this novel supply architecture and derives requirements for the regulatory-compliant design and commissioning of DC-coupled bidirectional direct current (DC) charge points (CPs). Furthermore, a proposal for the selection, dimensioning and interconnection of the electrical components, as well as the associated control and communication architecture of a corresponding power scalable charger is presented. This also includes the meaningful integration into an industrial DC grid, including the backend communication with the associated energy management system (EMS), which also plays a decisive role in the exploitation of the potential of the technology. In the first test phase of the charging station, a power-hardware-in-The-loop EV simulation will be carried out in conjunction with a regeneratively fed industrial low voltage direct current grid until standardized solutions for bidirectional EVs are actually available on the market.
CRIS-Team Scopus Importer:2024-08-02
Keywords: bidirectional charging; Direct current grid; LVDC; smart grid; V2G

Bromination of 2D materials - Freiberger, Eva Marie; Steffen, Julien; Waleska-Wellnhofer, Natalie; Hemauer, Felix; Schwaab, Valentin; Görling, Andreas; Steinrück, Hans-Peter; Papp, Christian (Nanotechnology, 2024)
The adsorption, reaction and thermal stability of bromine on Rh(111)-supported hexagonal boron nitride (h-BN) and graphene were investigated. Synchrotron radiation-based high-resolution x-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS allowed us to follow the adsorption process and the thermal evolution in situ on the molecular scale. On h-BN/Rh(111), bromine adsorbs exclusively in the pores of the nanomesh while we observe no such selectivity for graphene/Rh(111). Upon heating, bromine undergoes an on-surface reaction on h-BN to form polybromides (170-240 K), which subsequently decompose to bromide (240-640 K). The high thermal stability of Br/h-BN/Rh(111) suggests strong/covalent bonding. Bromine on graphene/Rh(111), on the other hand, reveals no distinct reactivity except for intercalation of small amounts of bromine underneath the 2D layer at high temperatures. In both cases, adsorption is reversible upon heating. Our experiments are supported by a comprehensive theoretical study. DFT calculations were used to describe the nature of the h-BN nanomesh and the graphene moiré in detail and to study the adsorption energetics and substrate interaction of bromine. In addition, the adsorption of bromine on h-BN/Rh(111) was simulated by molecular dynamics using a machine-learning force field.
CRIS-Team Scopus Importer:2024-02-09
Keywords: bromine; functionalization; graphene; hexagonal boron nitride; machine-learning force field; molecular dynamics; x-ray photoelectron spectroscopy

Carbon/graphene quantum dots as electrolyte additives for batteries and supercapacitors: A review - Shaker, Majid; Ng, Siow Woon; Sadeghi Ghazvini, Ali Asghar; Javanmardi, Shayan; Gaho, Mehran Ali; Jin, Zhan; Ge, Qi (Journal of Energy Storage, 2024)
Electrolyte is a crucial component in electrochemical energy storage devices (EESDs) such as rechargeable batteries and supercapacitors (SCs). While a substantial effort has been directed to engineering the electrodes, fewer but effective and continuous efforts have been devoted to improve the devices' performance through the modification of their electrolytes. Among these approaches, the strategic integration of minor amounts of additives has led to the formation of appropriate solid-electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI), as well as modifying the ionic conductivity, preventing dendrite growth, and improving charge storage kinetics. Among various additives in the electrolytes, carbon quantum dots (CDs) and graphene quantum dots (GQDs) have emerged as favorable options due to their positive influence on the electrolytes for EESDs. These QDs are soluble in specific solvents and their numerous functional moieties allow them to act as electrolyte additives and polyanions. Furthermore, their excellent physiochemical properties make them excellent candidates as fillers in solid-state electrolytes. The excellence of these QDs is verified by numerous electrochemical and physiochemical characterization methods affirming their positive influence on various parameters such as ion-diffusion kinetics, capacity, and resistance. The application of CDs and GQDs in the electrolytes of EESDs is new and the ruling mechanisms are not well-known yet. This review article provides an overview of the application of CDs and GQDs in both liquid and solid electrolytes for lithium-, sodium-, potassium-, zinc-ion and lithium metal batteries besides SCs. Herein, we aim to offer valuable insights into designing and fabricating a new class of electrolyte additives based on CDs and GQDs for realizing novel electrolytes for EESDs.
CRIS-Team Scopus Importer:2024-03-15
Keywords: Battery; Carbon dots; Electrolyte additives; Graphene quantum dots; Supercapacitor

Carbon nano-onions: Individualization and enhanced water dispersibility - Lucherelli, Matteo Andrea; Stiegler, Lisa; Steiger, Florian; Åhlgren, E. Harriet; Requena-Ramírez, Julia; Castro, Edison; Echegoyen, Luis; Hirsch, Andreas; Peukert, Wolfgang; Kotakoski, Jani; Walter, Johannes; Perez-Ojeda Rodriguez, Maria Eugenia; et al. (Carbon, 2024)
Carbon nano-onions (CNOs) are a unique class of carbon nanomaterials, with a concentric fullerene-like structure and sp2-hybridized carbon atoms. Onions present high mechanical strength, good conductivity, the capability to intercalate alkali metals, showing promising applications in diverse fields, such as tribology, energy storage, and nanomedicine. Their production has been developed by several techniques, with the most interesting being the thermal annealing of carbon nanodiamonds because of the easy scalability (gram-scale) and morphologic control (size and shape form perfectly spherical to polygonal). However, this synthesis is commonly affected by the formation of strong aggregates larger than 100 nm, attributed to carbon soot formation or strong van der Waals interactions, hindering the work with small and individual particles, leading to an altered yield of functionalization. In this study we propose a method for CNO individualization based on strong acid treatment, yielding highly water dispersible nanoparticles. Analytical ultracentrifugation (AUC) analysis is employed as a technique to investigate the particle dimensions directly in dispersion, permitting an ensemble analysis free from further sample preparation bias. Moreover, we also provide a comprehensive evaluation of the common post-synthetic treatment methods, and their effects. By means of scanning transmission electron microscopy with medium angle annular dark field detector and electron energy loss spectroscopy (STEM-MAADF and EELS) we have shown images of individual CNOs. Their successful separation achieved in this study is significant for future research and applications in nanomedicine, electrochemistry, and materials composites, where sample homogeneity is critical.<br
CRIS-Team Scopus Importer:2024-01-12
Keywords: Analytical ultracentrifugation; Carbon materials; Carbon nano-onions; Exfoliation; Materials chemistry

Carrier Transport in a Deep Ultraviolet Mixed Quantum Well Light Emitting Diode - Römer, Friedhard; Hofmann, Gregor; Hoopfner, Jakob; Schilling, Marcel; Muhin, Anton; Wernicke, Tim; Kneissl, Michael; Witzigmann, Bernd (IEEE Photonics Journal, 2024)
Aluminium Gallium Nitride (AlGaN) based light emitting diodes (LED) are the enabling technology for compact emitters of deep ultraviolet (DUV) radiation and are in high demand for environmental and medical applications. The efficiency of recent DUV LEDs is in the range of a few percent providing some potential for improvement. Apart from the light extraction efficiency the hole injection into the active region presents a major obstacle towards more efficient DUV LEDs. In this work, we investigate the emission spectra of a mixed multi quantum well (MQW) DUV LED to attain details on the active region carrier transport that allow an improvement of the hole injection. Changing the position of the long wavelength marker quantum well yields characteristic emission spectra which have been modelled with a multi scale carrier transport and luminescence simulator. The numerical modelling enables the extraction of opaque carrier transport characteristics in AlGaN such as the hole mobility in the highly doped barriers of the MQW.
CRIS-Team Scopus Importer:2024-01-26
Keywords: Aluminum gallium nitride; carrier transport; Charge carrier processes; deep ultraviolet; III-Nitride; Light emitting diode; Light emitting diodes; Mathematical models; numerical modelling; Quantum well devices; Radiative recombination; Wide band gap semiconductors

Combined continuous nanoparticle synthesis with chromatographic size classification - Gromotka, Lukas; Biegel, Markus; Damm, Cornelia; Traore, Nabi; Drobek, Dominik; Spiecker, Erdmann; Peukert, Wolfgang (Advanced Powder Technology, 2024)
In this paper, we report a combination of the continuous flow synthesis of gold nanoparticles (AuNPs) with subsequent purification and narrowing of the particle size distribution (PSD) by size-exclusion chromatography (SEC) by adapting the flow rates of synthesis and classification. First, we show scalability of chromatographic classification with respect to column dimension and the absence of irreversible nanoparticle adhesion on the column material. Two different syntheses lead to a large and widely distributed and a small and narrowly distributed AuNP dispersion, which are classified by a semipreparative column. The PSDs of individual fractions are characterized by analytical SEC. The broadly distributed AuNP dispersion was classified into three fractions with distinct PSDs. For the narrowly distributed AuNPs, the separation is almost independent of the mobile phase flow rate: coarse and fine fractions with almost identical PSDs and separation efficiency curves are observed irrespective of the flow rate. Even NP samples with narrow PSDs can be classified into multiple fractions with tailored PSDs while simultaneously removing dissolved impurities from the dispersion. With our study, we demonstrate the potential of a direct combination of continuous NP synthesis with chromatographic classification for the optimization of final PSDs and the simultaneous purification of nanoparticulate dispersions.
CRIS-Team Scopus Importer:2023-12-15
Keywords: Continuous flow synthesis; Gold nanoparticles; Purification; Size classification; Size-exclusion chromatography

Comparative Experimental Evaluation of a Three-Phase ARCP Inverter With a Single Shared Inductor Using SiC MOSFETs and Si IGBTs - Lehmeier, Thomas; Amler, Adrian; Zhou, Yan; März, Martin (IEEE Applied Power Electronics Conference & Exposition (APEC 2024), 2024)
Using soft-switching, the efficiency and EMI performance of power inverters for drive and grid applications can be improved. An advantageous topology is the well-known auxiliary resonant commutated pole (ARCP) inverter. The novel single shared inductor S2I-ARCP is a modification of the conventional ARCP which saves two resonant inductors in the auxiliary circuit in case of a three-phase inverter application. This paper compares the use of SiC MOSFETs with Si IGBTs as main switches and the possible gain in efficiency compared to hard-switched implementation. The S 2 I-ARCP topology is implemented with a suitable control algorithm and tested at up to 15 kW with an 800 V dc-link. It is shown that at a switching frequency of 30 kHz, soft-switching can reduce losses by approximately 50% for configurations using either SiC MOSFETs or Si IGBTs as main switches. For both cases, the efficiency peaks at 99.65% and 98%, respectively.<br

Comparative Study on Different Methods to Generate Synthetic Data for the Classification of THT Solder Joints - Thielen, Nils; Rachinger, Ben; Schröder, Felix; Preitschaft, Anja; Meier, Sven; Seidel, Reinhardt; Reinhardt, Andreas; Franke, Jörg (2024 1st International Conference on Production Technologies and Systems for E-Mobility (EPTS), 2024)
Automated Optical Inspection (AOI) is still one of the major tools for the quality control of solder joints; especially, if the requirements for the solder joints' mechanical and electrical properties are high. This is the case for products in the automotive sector, e.g. air bags or brake systems. Conventional test routines are time-consuming to define and not flexible regarding quality limits and lead to a high amount of manual reinspection. Thus, Artificial Intelligence offers the potential to address these disadvantages. Artificial Intelligence, on the other hand, suffers from highly imbalanced datasets resulting from the low amount of defects on solder joint level. One possibility to face this challenge is the utilization of synthetic data. This work compares different methods to generate data using both conventional data augmentation and deep generative models as well as virtual world rendering in order to enhance dataset quality in general. These are applied to an industrial dataset of THT solder joints. All tested approaches lead to an improvement in model quality. The best results are achieved with synthetic images from a Generative Adversarial Network, relatively increasing performance by around 6 % while minimizing error slip.<br
Keywords: Safety devices; Production; Quality control; Optical imaging; Rendering (computer graphics); Adaptive optics; Mechanical factors; Synthetic Data; Artificial Intelligence; Automated Optical Inspection; Electronics Production

Conformational Landscapes and Energetics of Carbon Nanohoops and their Ring-in-Ring Complexes - Geue, Niklas; Freiberger, Markus; Frühwald, Stefan; Görling, Andreas; Drewello, Thomas; Barran, Perdita E. (Journal of Physical Chemistry Letters, 2024)
Carbon nanohoops are promising precursors for the synthesis of nanotubes, whose structural dynamics are not well understood. Here, we investigate the conformational landscape and energetics of cycloparaphenylenes (CPPs), a methylene-bridged CPP and a carbon nanobelt. These nanohoops can form host-guest complexes with other rings, and understanding their structure is crucial for predicting their properties and identifying potential applications. We used a combination of ion mobility, tandem mass spectrometry, and density functional theory to characterize the nanohoops and their ring-in-ring complexes, following the energetics and conformations of their disassembly from intact complexes to fragment ions. Our results show structural integrity of the nanohoops and host-guest complexes. They also reveal interesting trends in size, packing density, stability, and structure between [6]CPP, the methylene-bridged CPP, and the carbon nanobelt as guests in ring-in-ring complexes. Taken together, our work illustrates how mass spectrometry data can help to unravel the rules that govern the formation of carbon nanohoop assemblies.
CRIS-Team Scopus Importer:2024-07-05

Controlling Interchromophore Coupling in Diamantane-Linked Pentacene Dimers To Create a “Binary” Pair - Greißel, Phillip; Schroeder, Zachary W.; Thiel, Dominik; Ferguson, Michael J.; Clark, Timothy; Guldi, Dirk Michael; Tykwinski, Rik R. (Journal of the American Chemical Society, 2024)
Two isomeric pentacene dimers, each linked by a diamantane spacer, have been synthesized. These dimers are designed to provide experimental evidence to support quantum mechanical calculations, which predict the substitution pattern on the carbon-rich diethynyldiamantane spacer to be decisive in controlling the interpentacene coupling. Intramolecular singlet fission (i-SF) serves as a probe for the existence and strength of the electronic coupling between the two pentacenes, with transient absorption spectroscopy as the method of choice to characterize i-SF. 4,9-Substitution of diamantane provides a pentacene dimer (4,9-dimer) in which the two chromophores are completely decoupled and that, following photoexcitation, deactivates to the ground state analogous to a monomeric pentacene chromophore. Conversely, 1,6-substitution provides a pentacene dimer (1,6-dimer) that exhibits sufficiently strong coupling to drive i-SF, resulting in correlated triplet M(T1T1) yields close to unity and free triplet (T1 + T1) yields of ca. 50%. Thus, the diamantane spacer effectively switches “on” or “off” the coupling between the chromophores, based on the substitution pattern. The binary control of diamantane contrasts other known molecular spacers designed only to modulate the coupling strength between two pentacenes.
CRIS-Team Scopus Importer:2024-04-19

Controlling the Morphology in Electrostatic Self-Assembly via Light - Agarwal, Mohit; Zika, Alexander; Schweins, Ralf; Gröhn, Franziska (Polymers, 2024)
Electrostatic self-assembly of macroions is an emerging area with great potential in the development of nanoscale functional objects, where photo-irradiation responsiveness can either elevate or suppress the self-assembly. The ability to control the size and shape of macroion assemblies would greatly facilitate the fabrication of desired nano-objects that can be harnessed in various applications such as catalysis, drug delivery, bio-sensors, and actuators. Here, we demonstrate that a polyelectrolyte with a size of 5 nm and multivalent counterions with a size of 1 nm can produce well-defined nanostructures ranging in size from 10–1000 nm in an aqueous environment by utilizing the concept of electrostatic self-assembly and other intermolecular non-covalent interactions including dipole–dipole interactions. The pH- and photoresponsiveness of polyelectrolytes and azo dyes provide diverse parameters to tune the nanostructures. Our findings demonstrate a facile approach to fabricating and manipulating self-assembled nanoparticles using light and neutron scattering techniques.
CRIS-Team Scopus Importer:2024-01-19
Keywords: electrostatic assembly; kinetically controlled; light irradiation; nanostructures; polyelectrolyte; thermodynamically controlled

Correlations in strong-field-emitted ultrashort electron pulses from metal needle tips - Meier, Stefan; Heimerl, Jonas; Hommelhoff, Peter (Laser Physics Letters, 2024)
When two electrons are emitted from a metal needle tip with the help of femtosecond laser pulses, they show a strong anticorrelation signal in the energy domain. Depending on the wavelength and intensity of the driving laser pulses, the electron emission process can be either in a perturbative regime, like single- or multi-photon photoemission, or in the strong-field regime, where emission is dominated by the instantaneous electric field of the laser pulse, or in the intermediate regime. Here, we report on the two-electron anticorrelation signal and how it evolves from the multiphoton toward the strong-field emission regime. We show that in both cases, the resulting anticorrelation signal can be well explained by semi-classical simulations using a point-particle model, thus the dynamics is dominated by the center-of-mass dynamics of the individual electrons. However, the actual emission process of multiple interacting electrons requires improved quantum mechanical models and therefore remains the subject of future work. This paper is part of the Special Topic Collection: papers from the 31th Annual International Laser Physics Workshop 2023 (LPHYS 2023

Creep in 0.91(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.03K0.5Na0.5NbO3 lead-free piezoceramic under constant electric field loads - Chen, Di; Ayrikyan, Azatuhi; Chu, Xiangcheng; Kamlah, Marc; Webber, Kyle Grant (Journal of Physics D: Applied Physics, 2024)
This work explores the creep behavior of polycrystalline 0.91(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.03K0.5Na0.5NbO3 under constant electric fields. It reveals intriguing time-dependent variations in both polarization and strain response, which can be attributed to a transformation from the relaxor state to a long-range ferroelectric order. Meanwhile, bulk volume resistivity values are obtained to eliminate the influence of leakage current on the polarization assessment. The findings provide valuable insights into the creep behavior of lead-free relaxor ferroelectrics, laying a solid foundation for enhancing the performance and reliability of piezoactuators.
CRIS-Team Scopus Importer:2023-12-01
Keywords: constant electric field loading; creep behavior; lead-free piezoceramic

C[sbnd]C bond cleavage in the electrooxidation of 2,3-butanediol controlled by an ionic liquid modifier - Yang, Juntao; Haßfurther, Florian; Hilpert, Felix; Hussain, Zarah; Yang, Tian; Taccardi, Nicola; Wasserscheid, Peter; Brummel, Olaf; Libuda, Jörg (Journal of Catalysis, 2024)
In heterogeneous catalysis, ionic liquids (ILs) are used as chemical modifiers to control selectivity. In our work, we aim to apply the same concept to electrocatalytic systems. As a model reaction, we studied the electrooxidation of 2,3-butanediol on the low-index Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) surfaces in an acidic environment. We used the IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C2C1Im][OTf]) dissolved in an aqueous electrolyte as a catalyst modifier. The reaction mechanisms were investigated by electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). The oxidation of 2,3-butanediol is highly structure dependent. On all three surfaces, the two products formed are acetoin and diacetyl, i.e. either one or two alcohol functionalities are oxidized. However, we observe distinct features on the different surfaces with respect to activity, potential window of oxidation, and selectivity. Only the Pt(1 0 0) surface is active towards C[sbnd]C bond cleavage. The latter reaction leads to the formation of COads and poisoning of the catalyst. Modification of this surface by addition of the IL leads to an increase of the selectivity for acetoin from 51 % to 78 % (at 1.1 VRHE). In addition, C[sbnd]C bond cleavage is suppressed, no CO is formed, and the surface remains active for the target reaction. We attribute these effects to the reversible and structure dependent adsorption of the [OTf]− anions on the Pt surfaces and additional interionic interactions. Our results demonstrate the potential of ILs to control selectivity in electrocatalytic reactions.
CRIS-Team Scopus Importer:2024-05-31
Keywords: In-situ IR spectroscopy; Ionic liquids; Platinum; Secondary alcohols; Selectivity; Single crystals; Supported catalyst with ionic liquid layer

Designing Manual Workplace Systems in Engineer-to-Order Enterprises to Improve Productivity: A Kano Analysis - Stoidner, Micha; Bründl, Patrick; Matthes, Tina; Nguyen, Huong Giang; Abrass, Ahmad; Franke, Jörg (SAE International Journal of Materials and Manufacturing, 2024)
Being an engineer-to-order (ETO) operating industry, the control cabinet industry faces difficulties in process and workplace optimizations due to changing requirements and lot size one combined with volatile orders. To optimize workplaces for employees, current literature is focusing on ergonomic designs, providing frameworks to analyze workplaces, leaving out the optimal design for productivity. This work thus utilizes a Kano analysis, collecting empirical data to identify essential design requirements for assembly workplaces, incorporating input from switchgear manufacturing employees. The results emphasize the need for a balance between ergonomics and efficiency in workplace design. Surprisingly, few participants agree on the correlation between improved processes and workspaces having a positive impact on their well-being and product quality. Ultimately, the study offers a list of requirements that are needed at ETO assembly stations to satisfy employees and improve efficiency of the production processes.<br
Keywords: Aerospace; Automotive; Commercial Vehicle; Optimization; Productivity; Personnel; Design processes; Supply chain management; Ergonomics; Production; Product development

Determination of the fundamental absorption and optical bandgap of dielectric thin films from single optical transmittance measurements - Tejada, A.; Montañez, L.; Torres, C.; Llontop, P.; Flores, L.; de Zela, F.; Winnacker, Albrecht; Guerra, J. A. (Applied Optics, 2019)
In this work, we propose a method to retrieve the thickness and optical constants of dielectric thin films from single optical transmittance measurements. The method is based on the envelope method and requires a simple dispersion model for the real part of the refractive index with few fitting parameters, while the absorption coefficient can be determined without the aid of a dispersion model. The wavelength-dependent optical constants can be obtained even from spectra that exhibit few interference fringes. We have tested the method with simulated and real transmittance data from thin films in the spectral range covering the fundamental absorption. In order to assess the method’s reliability to retrieve the optical constants and optical bandgap, a comparison is performed with the method by Chambouleyron, known as the Pointwise Unconstrained Minimization Approach, and a fit using the Cody–Lorentz dispersion model. We evaluate the methods’ capability to retrieve the fundamental absorption and optical bandgap, and their compromise with film thickness accuracy. Finally, the methods are tested and contrasted using optical transmittance of three different semiconductor material thin films.
CRIS-Team Scopus Importer:2019-12-20

Dewetting of Pt Nanoparticles Boosts Electrocatalytic Hydrogen Evolution Due to Electronic Metal-Support Interaction - Harsha, Shreyas; Sharma, Rakesh K.; Dierner, Martin; Baeumer, Christoph; Makhotkin, Igor; Mul, Guido; Ghigna, Paolo; Spiecker, Erdmann; Will, Johannes; Altomare, Marco (Advanced Functional Materials, 2024)
Solid-state dewetting is the heat-induced agglomeration of thin metal films into defined nanoparticles (NPs). Dewetted Pt nanoparticles are investigated on F-doped SnO2 (FTO) substrates as model binder-free electrodes for the hydrogen evolution reaction (HER). Dewetting of Pt films into particles exposes the FTO substrate and the metal/support (Pt-FTO) contact line. Despite the decrease in Pt electrochemical surface area (ECSA) upon dewetting, dewetted NPs show a >3-fold increase in ECSA-normalized HER activity compared to as-deposited nanocrystalline Pt films. Electrodes designed with dewetted Pt NPs of different sizes show that the HER activity does not only correlate with the ECSA but also increases with increasing the Pt-FTO contact line length. The smaller the NPs, the larger the Pt-FTO contact line, and the higher the activity. This effect is ascribed to electronic metal-support interaction (EMSI), due to electron transfer from FTO to Pt. It is proposed that EMSI effects alter the electronic structure of Pt sites near the Pt-FTO contact line, facilitating the H2 evolution kinetics. When NPs are a few nm-sized, a large mass fraction of Pt is affected by EMSI, resulting in a further increase of HER activity compared to NPs ≥10 nm despite the lower ECSA.
CRIS-Team Scopus Importer:2024-08-23
Keywords: electrocatalysis; electronic metal-support interaction; hydrogen evolution reaction; nanoparticles; platinum; solid-state dewetting

Effect of Li on the intrinsic and extrinsic contributions of the piezoelectric response in Lix(Na0.5K0.5)1-xNbO3 piezoelectric ceramics across the polymorphic phase boundary - Martin, Alexander; Khansur, Neamul Hayet; Urushihara, Daisuke; Asaka, Toru; Kakimoto, Ken ichi; Webber, Kyle Grant (Acta Materialia, 2024)
The temperature-dependent Rayleigh behavior of polycrystalline lead-free Lix(Na0.5K0.5)1-xNbO3 ceramics was characterized to measure the reversible and irreversible piezoelectric response of different compositions with varying Li content. The results revealed that all compositions showed peak values of both reversible and irreversible piezoelectric response near the polymorphic phase boundary. Additionally, depending on the crystal structure, Li content affected the irreversible and reversible contributions. Specifically, at room temperature, there was a notable increase in the irreversible contribution with rising Li content, whereas, within the tetragonal region, the change in Li content predominantly influenced the reversible contributions, resulting in their reduction. These results are discussed in conjunction with temperature-dependent X-ray diffraction and transmission electron microscopy data, which indicated that the lattice distortion and domain structure play an important role in the temperature-dependent variation in the piezoelectric response.
CRIS-Team Scopus Importer:2024-02-16
Keywords: Lead-free piezoceramics; Polymorphic phase boundary; Rayleigh behavior

Efficient Training of Recurrent Neural Networks for Remaining Time Prediction in Predictive Process Monitoring - Roider, Johannes; Zanca, Dario; Eskofier, Björn (Business Process Management, 22nd International Conference, BPM 2024, Krakow, Poland, September 1–6, 2024, Proceedings, 2024)
An important task in predictive process monitoring is the prediction of the remaining time. The accuracy of methods to solve this task has been steadily improved and verified during the last years. The current state-of-the-art uses Long Short-Term Memory (LSTM) neural networks, represented by the DA-LSTM model. However, training such methods requires substantial amounts of time and memory. This paper addresses specifically these problems. We adjust DA-LSTM and introduce DA-LSTM+ which achieves competitive error levels while reducing the training time significantly. Furthermore, we introduce trace-based sequence encoding as an alternative to prefix encoding, and an approach to use case attributes more efficiently to address time and memory limitations during training. The usage of these is not limited to LSTM’s but they are compatible with any neural network type. We evaluate them together with two alternatives for categorical feature encoding in an extensive benchmark study including eight different model architectures based on DA-LSTM+ and 14 publicly available datasets. The study shows that the training of neural network-based methods can be significantly accelerated with our contributions without affecting model’s performance. Our implementation is memory-efficient and publicly availabl
Keywords: Predictive Process Monitoring; Deep Learning; Process Mining; Remaining Time Prediction

Enhanced LWIR response of InP/InAsP quantum discs-in-nanowire array photodetectors by photogating and ultra-thin ITO contacts - Jeddi, Hossein; Adham, Kristi; Zhao, Yue; Witzigmann, Bernd; Römer, Friedhard; Bermeo, Marie; Borgström, Magnus T.; Pettersson, Håkan (Nanotechnology, 2024)
Here we report on an experimental and theoretical investigation of the long-wavelength infrared (LWIR) photoresponse of photodetectors based on arrays of three million InP nanowires with axially embedded InAsP quantum discs. An ultra-thin top indium tin oxide contact combined with a novel photogating mechanism facilitates an improved LWIR normal incidence sensitivity in contrast to traditional planar quantum well photodetectors. The electronic structure of the quantum discs, including strain and defect-induced photogating effects, and optical transition matrix elements were calculated by an 8-band k·p simulation along with solving drift-diffusion equations to unravel the physics behind the generation of narrow linewidth intersubband signals observed from the quantum discs.
CRIS-Team Scopus Importer:2024-07-19
Keywords: infrared photodetectors; long-wavelength infrared (LWIR); nanowire array photodetectors; nanowires; photogating; quantum discs-in-nanowires; ultra-thin ITO contacts

Enhancing Flexibility in Power Electronics Production: A Study on Robotic Handling of Copper Clips for Laser Bonding - Ockel, Manuela; Janisch, Lucas; Franke, Jörg; Schlichting, Johannes; Breuer, Marius (11th International Conference on Control, Mechatronics and Automation, ICCMA 2023, 2023)
Keywords: semiconductor device packaging; power electronics; robotics; laser bonding; gripper

Enhancing unsupervised outlier model selection: A study on ireos algorithms - Schlieper, Philipp; Luft, Hermann; Klede, Kai; Strohmeyer, Christoph; Eskofier, Björn; Zanca, Dario (ACM Transactions on Knowledge Discovery from Data, 2024)
Outlier detection stands as a critical cornerstone in the field of data mining, with a wide range of applications spanning from fraud detection to network security. However, real-world scenarios often lack labeled data for training, necessitating unsupervised outlier detection methods. This study centers on Unsupervised Outlier Model Selection (UOMS), with a specific focus on the family of Internal, Relative Evaluation of Outlier Solutions (IREOS) algorithms. IREOS measures outlier candidate separability by evaluating multiple maximum-margin classifiers and, while effective, it is constrained by its high computational demands. We investigate the impact of several different separation methods in UOMS in terms of ranking quality and runtime. Surprisingly, our findings indicate that different separability measures have minimal impact on IREOS' effectiveness. However, using linear separation methods within IREOS significantly reduces its computation time. These insights hold significance for real-world applications where efficient outlier detection is critical. In the context of this work, we provide the code for the IREOS algorithm and our separability techniques.
CRIS-Team Scopus Importer:2024-07-05
Keywords: anomaly detection; model selection; Outlier detection; unsupervised evaluation

Estimating 3D kinematics and kinetics from virtual inertial sensor data through musculoskeletal movement simulations - Nitschke, Marlies; Dorschky, Eva; Leyendecker, Sigrid; Eskofier, Björn; Koelewijn, Anne (Frontiers in Bioengineering and Biotechnology, 2024)
Portable measurement systems using inertial sensors enable motion capture outside the lab, facilitating longitudinal and large-scale studies in natural environments. However, estimating 3D kinematics and kinetics from inertial data for a comprehensive biomechanical movement analysis is still challenging. Machine learning models or stepwise approaches performing Kalman filtering, inverse kinematics, and inverse dynamics can lead to inconsistencies between kinematics and kinetics. We investigated the reconstruction of 3D kinematics and kinetics of arbitrary running motions from inertial sensor data using optimal control simulations of full-body musculoskeletal models. To evaluate the feasibility of the proposed method, we used marker tracking simulations created from optical motion capture data as a reference and for computing virtual inertial data such that the desired solution was known exactly. We generated the inertial tracking simulations by formulating optimal control problems that tracked virtual acceleration and angular velocity while minimizing effort without requiring a task constraint or an initial state. To evaluate the proposed approach, we reconstructed three trials each of straight running, curved running, and a v-cut of 10 participants. We compared the estimated inertial signals and biomechanical variables of the marker and inertial tracking simulations. The inertial data was tracked closely, resulting in low mean root mean squared deviations for pelvis translation (≤20.2 mm), angles (≤1.8 deg), ground reaction forces (≤1.1 BW%), joint moments (≤0.1 BWBH%), and muscle forces (≤5.4 BW%) and high mean coefficients of multiple correlation for all biomechanical variables (≥0.99) . Accordingly, our results showed that optimal control simulations tracking 3D inertial data could reconstruct the kinematics and kinetics of individual trials of all running motions. The simulations led to mutually and dynamically consistent kinematics and kinetics, which allows researching causal chains, for example, to analyze anterior cruciate ligament injury prevention. Our work proved the feasibility of the approach using virtual inertial data. When using the approach in the future with measured data, the sensor location and alignment on the segment must be estimated, and soft-tissue artifacts are potential error sources. Nevertheless, we demonstrated that optimal control simulation tracking inertial data is highly promising for estimating 3D kinematics and kinetics for a comprehensive biomechanical analys

Evaluation of Semiconductor-Based Isolation for Electric Vehicle Chargers in DC Microgrids - Drexler, Kilian; Gehring, Johannes; Wunder, Bernd; Zhou, Yan; März, Martin; Lorentz, Vincent R. H. (2024 IEEE Sixth International Conference on DC Microgrids (ICDCM), 2024)
In this paper, converters with semiconductor-based isolation for the application of electric vehicle (EV) charging in DC-microgrids are introduced. A PLECS circuit simulation model is set up to examine the fault behavior of those converters in comparison to standard isolated and non-isolated topologies. Three different fault scenarios are evaluated and influence of the converter topology on the system behavior is examined. It is shown that the semiconductor-based isolating converter has a similar behavior to the standard isolating converter considering systemic fault
Keywords: Semiconductor device modeling; Circuit simulation; Microgrids; Electric vehicle charging; Topology; Circuit faults; Integrated circuit modeling; DC grid; electric vehicle chargers; semiconductor-based isolation; converter topologies; fault behavior; ground fault; simulation

Evaluation of the Cold Atmospheric Plasma Metallization of Bare Dies with Copper Through Life Cycle Assessment - Ockel, Manuela; Funk, Felix; Janisch, Lucas; Franke, Jörg (Production at the Leading Edge of Technology. Proceedings of the 13th Congress of the German Academic Association for Production Technology (WGP), Freudenstadt, November 2023, 2024)
The cold atmospheric plasma metallization (CAPM) with copper is an alternative coating process to electroplating for applying a thermo-mechanical buffer onto bare dies to enable more powerful top-side interconnection technologies for power electronics. A complete life cycle assessment (LCA) is to be performed in order to evaluate improvements to increase sustainability of the copper metallization. Since life cycle assessment results strongly depends on the set system boundaries and availability of primary data, the scope of this study is determined by the availability of the latter. Since the coating properties are dependent on the plasma spray parameters, a variation of set parameters is assessed, which are currently used for the coating of the bare dies. The evaluation showed that electricity, plasma gas and powder production have major impacts on the sustainability. The necessary heat treatment is responsible for on third of the emissions if bare die production and plasma coating are included into the assessment. Potential improvements are the reduction of total conveyed powder and carrier gas, as well as increasing the number of bare dies treated in one annealing process.
CRIS-Team Scopus Importer:2023-12-15
Keywords: Cold Atmospheric Plasma Metallization; Copper; Life Cycle Assessment; Power Electronics

Examining the dynamics of bargaining power in buyer-supplier negotiations: insights from a case study in the German automotive industry - Bodendorf, Frank; Franke, Jörg (International Journal of Production Research, 2024)
Buyer-supplier negotiations play a central role in supply chain networks regulating the conditions for the exchange of goods. This paper investigates the relationships between influencing factors and their impact on bargaining power. In a multiple case study, we triangulate secondary data from 12 German automotive suppliers. Following a structural equation modelling approach, bargaining power and the effects of those influencing variables are analysed. Four relationships can be found. (1) The availability of internal resources and the existence of strong buyer-supplier relationships have a positive effect on bargaining power. (2) A positive influence of buyer-supplier relationships on gaining access to internal resources can be identified. (3) External market conditions have a small negative effect on bargaining power. (4) No significant effect can be found looking at the impact of external market conditions on internal resources or buyer-supplier relationships. The findings support theory by showing that internal resources and strong buyer-supplier relationships positively influence bargaining power. Based on these results, managers are advised to prioritise strategic resource allocation, implement win-win strategies, and employ benchmarking techniques to enhance competitiveness and strengthen bargaining positions.
CRIS-Team Scopus Importer:2024-01-26
Keywords: Bargaining power; buyer-supplier relationships; market conditions; negotiation; resources

Experimental Investigations in the Processing of AISI H11 Powder Blends Enriched with Tungsten Carbide Nanoparticles for the Additive Manufacturing of Tailored Hot Working Tools in the Directed Energy Deposition (DED-LB/M)—Impact of Tungsten Carbide Nanoparticles on Microstructural and Mechanical Characteristics - Hentschel, Oliver; Kohlstruck, Jan; Vetter, Johannes; Wittmann, Alexander; Krakhmalev, Pavel; Nikas, Dimitrios; Schmidt, Michael (Metals, 2024)
In this study, the DED-LB/M process of AISI H11 tool steel powder blends modified by adding WC nanoparticles (WC-np) in concentrations of 1, 2.5 and 5 wt.-% was the object of scientific investigations. For this, 30-layer cuboid specimens were manufactured. The overall scientific aim was to examine how the WC-np interact with the steel melt and in the end, influence the processability, microstructure and mechanical properties of produced specimens. The examinations were carried out on both as-built and thermally post-processed specimens. An advanced microstructural analysis (SEM, EDS, EBSD and XRD) revealed that due to the high solubility of WC-np in the molten steel, most of the WC-np appear to have dissolved during the ongoing laser process. Furthermore, the WC-np favor a stronger distortion and finer grain size of martensite in the manufactured specimens. An increase in hardness from about 650 HV1 for the H11 specimen to 780 HV1 for the one manufactured using the powder blend containing 5 wt.-% of WC-np was observed in as-built conditions. In the same way, the compression yield strength enhanced from 1839 MPA to 2188 MPA. The hardness and strength increasing effect of WC-np remained unchanged even after heat treatments similar to those used in industry.
CRIS-Team Scopus Importer:2024-03-01
Keywords: Additive Manufacturing; AISI H11; bulk forming tools; Directed Energy Deposition (DED-LB/M); hot work tool steel; in situ alloying; nanoparticles; Tungsten Carbide (WC)

Exploration of Use-Case-Dependent Modeling Approach for Distributed DC-Grids - Lavery, Melanie; Schwanninger, Raffael; März, Martin (2024 IEEE Sixth International Conference on DC Microgrids (ICDCM), 2024)
In this paper, the previously described Use-Case-Dependent Modeling Approach is evaluated and its usefulness for grid simulation is explored. One of the main merits of the approach is its application in automated system analysis. Previous results also suggest a reduced usage of computing resources. This however has not yet been systematically tested and properly verified. Therefore, to inspect the validity of this approach, the methodology is explained and simulation results are compared to measurements of the modeled DC system. It is shown that the grid models adequately represent the system's behavior and the simulation results can therefore be used for system analysis. Discrepancies can be explained by an insufficient modeling of the measured system's damping elements. These differences are however small enough to be mitigated by usually applied error margins, especially for system stability analysis. Although more work is required for full verification, the validity and versatility of the approach is demonstrate

Feasibility of Powder-Based High-Throughput Synthesis for Ceramics Development: Case Study on the Influence of Calcination Temperature in BiFeO3-BaTiO3 - Eckstein, Udo; Kuhfuß, Michel; Fey, Tobias; Webber, Kyle Grant (Advanced Engineering Materials, 2024)
Recent advances in machine learning capabilities have increased interest in materials research to improve the efficiency of materials discovery and optimization as well as to better understand the underlying phenomena responsible for the observed physical properties. While combinatorial chemistry and compositional engineering is well established in the development of pharmaceutical and chemical products, its use in the field of bulk functional polycrystalline ceramics is far from mature. In this work, a critical review of a high-throughput powder-based dispensing system is provided and the challenges involved with the transition from a conventional, human resources intensive workflow to a fully automated process are highlighted. Based on the lead-free piezoelectric BiFeO3–BaTiO3 binary system, the applicability and robustness of high-throughput engineering is investigated to increase data point density in phase diagrams, including the composition variations of the resulting materials. This work presents 16 different BiFeO3–BaTiO3 compositions at four different calcination temperatures, both demonstrating the potential of the system. This is coupled to automated crystal structure analysis, which will be used to investigate the role of calcination temperature on the resulting compositions at room temperature.
CRIS-Team Scopus Importer:2024-03-01
Keywords: combinatorial chemistry; compositional engineering; high-throughput XRD; powder dispensing; solid state reactions; structural characterizations

Fleet & tail assignment under uncertainty - Glomb, Lukas; Liers-Bergmann, Frauke; Rösel, Florian (Discrete Optimization, 2024)
Airlines solve many different optimization problems and combine the resulting solutions to ensure smooth, minimum-cost operations. Crucial problems are the Fleet Assignment, which assigns aircraft types to flights of a given schedule, and the Tail Assignment, which determines individual flight sequences to be performed by single aircraft. In order to find a cost-optimal solution, many airlines use mathematical optimization models. For these to be effective, the available data and forecasts must reflect the situation as accurately as possible. However, especially in times of a pandemic, the underlying plan is subject to severe uncertainties: Staff and demand uncertainties can even lead to flight cancellations or result in entire aircraft having to be grounded. Therefore, it is advantageous for airlines to protect their mathematical models against uncertainties in the input parameters. In this work, two computational tractable and cost-efficient robust models and solution approaches are developed: First, we set up a novel mixed integer model for the integrated fleet and tail assignment, which ensures that as few subsequent flights as possible have to be canceled in the event of initial flight cancellations. We then solve this model using a procedure that ensures that the costs of the solution remain considerably low. Our second model is an extended fleet assignment model that allows us to compensate for entire aircraft cancellations in the best possible way, taking into account rescheduling options. We demonstrate the effectiveness of both approaches by conducting an extensive computational study based on real flight schedules of a major German airline. It turns out that both models deliver stable, cost-efficient solutions within less than ten minutes, which significantly reduce follow-up costs in the case uncertainties arise.<br
Keywords: (Mixed) Integer Linear Programming; Fleet assignment; Tail assignment; Robust Optimization

From Human to Robot Interaction towards Human to Robot Communication in Assembly Systems - Kambarov, Ikrom; Inoyatkhodjaev, Jamshid; Kunz, David; Brossog, Matthias; Franke, Jörg (Eurasia Proceedings of Science, Technology, Engineering and Mathematics, 2023)
The interaction between humans and robots has been a rapidly developing technology and a frequently discussed research topic in the last decade because current robots ensure the physical safety of humans during close proximity assembly operations. This interaction promises capability flexibility due to human dexterity skills and capacity flexibility due to robot accuracy. Nevertheless, in these interactions, the humans are marginally outside of the system, while the robots are seen as a crucial component of the assembly activities, which causes the systems to lack flexibility and efficiency. Therefore, this paper presents a study on Human to Robot communication in assembly systems. We conducted a systematic review of related literature and industrial applications involving human and robot interaction modes over the last decade to identify research gaps in the integration of collaborative robots into assembly systems. We believe that we are in a transformation phase from physical interaction mode towards cognitive interaction mode between humans and robots, where humans and robots are able to interact with each other during mutual working conditions and humans are able to guide robots. The main contribution of this paper is to propose a future mode of human-robot interaction in which a skilled operator performs not only physical cooperative tasks with robots but also work aided by smart technologies that allow communication with robots. This interaction mode allows for an increase in the flexibility and productivity of the assembly operation as well as t he wellbeing of the human operator in a human-centered manufacturing environment.
CRIS-Team Scopus Importer:2023-12-01
Keywords: Human robot communication; Human robot interaction; Industry 4.0; Industry 5.0

Graphene Oxide-BODIPY Conjugates as Highly Fluorescent Materials - Reina, Giacomo; Beneventi, Giovanni Mariano; Kaur, Ramandeep; Biagiotti, Giacomo; Cadranel, Alejandro; Ménard-Moyon, Cécilia; Nishina, Yuta; Richichi, Barbara; Guldi, Dirk Michael; Bianco, Alberto (Chemistry - A European Journal, 2023)
Covalent functionalization of graphene oxide (GO) with boron dipyrromethenes (BODIPYs) was achieved through a facile synthesis, affording two different GO-BODIPY conjugates where the main difference lies in the nature of the spacer and the type of bonds between the two components. The use of a long but flexible spacer afforded strong electronic GO-BODIPY interactions in the ground state. This drastically altered the light absorption of the BODIPY structure and impeded its selective excitation. In contrast, the utilisation of a short, but rigid spacer based on boronic esters resulted in a perpendicular geometry of the phenyl boronic acid BODIPY (PBA-BODIPY) with respect to the GO plane, which enables only minor electronic GO-BODIPY interactions in the ground state. In this case, selective excitation of PBA-BODIPY was easily achieved, allowing to investigate the excited state interactions. A quantitative ultrafast energy transfer from PBA-BODIPY to GO was observed. Furthermore, due to the reversible dynamic nature of the covalent GO-PBA-BODIPY linkage, some PBA-BODIPY is free in solution and, hence, not quenched from GO. This resulted in a weak, but detectable fluorescence from the PBA-BODIPY that will allow to exploit GO-PBA-BODIPY for slow release and imaging purposes.
CRIS-Team Scopus Importer:2023-05-19
Keywords: carbon nanomaterials; chemical functionalization; imaging; photoluminescent properties; transient absorption spectroscopy

Greening Regional Airports: A Vision for Carbon Neutral Infrastructure - Meindl, Markus; de Ruiter, Cor; Marciello, Valerio; Di Stasio, Mario; Saavedra-Rubio, Karen; Laurent, Alexis; Ruocco, Manuela; März, Martin (Contemporary Perspective on Science, Technology and Research Vol. 3, 2024)
This study provides an overview of the energy demand of a regional airport, divided into individual time horizons. The electrification of aircraft systems raises the question of whether airports will be among the largest electricity consumers in our infrastructure in the future. Sustainability and especially emission reductions are significant challenges for airports that are currently being addressed. The Clean Sky 2 project GENESIS addresses the environmental sustainability of hybrid-electric 50-passenger aircraft systems in a life cycle perspective to support the development of a technology roadmap for the transition to sustainable and competitive electric aircraft systems. This article originates from the GENESIS research and describes various options for ground power supply at a regional airport. Potential solutions for airport infrastructure with a short (2030), medium (2040), and long (2050) time horizon are proposed. In addition to the environmental and conservation benefits, switching from fossil fuels to sustainably produced fuels is also financially attractive for airports, airlines, and travelers. This analysis includes estimating the future energy demand per day, month, and year. In addition, the current flight plan based on conventional aircraft is adapted to the needs of a 50-PAX regional aircraft. The findings confirm that airports will require an enormous amount of electrical energy due to the electrification of air traffic.<br
Keywords: On-ground energy supply hybrid-electric aircraft airport infrastructure sustainable aviation

Guidelines for Material Design in Semitransparent Organic Solar Cells - Forberich, Karen; Troisi, Alessandro; Liu, Chao; Wagner, Michael; Brabec, Christoph J.; Egelhaaf, Hans Joachim (Advanced Functional Materials, 2024)
Organic solar cells (OSCs) are uniquely suited for semitransparent applications due to their adjustable absorption spectrum. However, most high-performance semitransparent cells reported to date are based on materials that have shown high power conversion efficiency for opaque devices. A model is therefore presented to assess the optimum efficiency and transparency for a specific donor and acceptor bandgap. The absorption characteristics of both donor and acceptor are modeled with spectral data of typical absorber materials from the literature which are adjusted to achieve the desired bandgap value. The results show three distinct regions of high light utilization efficiency (LUE) if the photopic curve is employed as a weighting function (corresponding to window applications), and a broad maximum for the plant action spectrum as a weighting function (corresponding to greenhouse applications). When comparing these findings to reported experimental values, it is evident that the bandgaps of the materials used for the experimental studies do not correspond to the maxima identified by the simulation model. The analysis of the energy levels of molecules recorded in the literature confirms that all bandgaps and therefore all LUE maxima are chemically feasible so that the performance of semitransparent OSCs can be further improved by designing materials with optimized absorption spectra.
CRIS-Team Scopus Importer:2024-03-22
Keywords: absorption spectrum; modeling; organic solar cells; Scharber plot; semitransparent

High-Precision Visual Servoing for the Neutron Diffractometer STRESS-SPEC at MLZ - Landesberger , Martin; Kedilioglu, Oguz; Wang , Lijiu; Gan , Weimin; Kornmeier , Joana Rebelo; Reitelshöfer, Sebastian; Franke, Jörg; Hofmann , Michael (Sensors, 2024)
With neutron diffraction, the local stress and texture of metallic components can be analyzed non-destructively. For both, highly accurate positioning of the sample is essential, requiring the measurement at the same sample location from different directions. Current sample-positioning systems in neutron diffraction instruments combine XYZ tables and Eulerian cradles to enable the accurate six-degree-of-freedom (6DoF) handling of samples. However, these systems are not flexible enough. The choice of the rotation center and their range of motion are limited. Industrial six-axis robots have the necessary flexibility, but they lack the required absolute accuracy. This paper proposes a visual servoing system consisting of an industrial six-axis robot enhanced with a high-precision multi-camera tracking system. Its goal is to achieve an absolute positioning accuracy of better than (Formula presented.) m. A digital twin integrates various data sources from the instrument and the sample in order to enable a fully automatic measurement procedure. This system is also highly relevant for other kinds of processes that require the accurate and flexible handling of objects and tools, e.g., robotic surgery or industrial printing on 3D surfaces.
CRIS-Team Scopus Importer:2024-05-24
Keywords: automation; computer vision; high accuracy; neutron diffraction; residual stress analysis; robotics; texture analysis; visual servoing

Holistic Concept for Simulation-based Planning and Design of Hybrid AC/DC Energy Grids for Production Systems - Barth, Martin; Gutwald, Benjamin; Russwurm, Eva; Lavery, Melanie; Schwanninger, Raffael; März, Martin; Franke, Jörg (Simulation Notes Europe, 2024)
This paper proposes a simulation-based planning concept and a simulation architecture for the design of hybrid AC/DC grids, which have the potential to significantly contribute to the energy transition. The paper discusses the lack of knowledge for the design of these grids, and presents a simulation approach to efficiently design hybrid grids and analyze them based on an electrical simulation. The authors also address the need to include process-specific characteristics in the planning and analysis of the electrical network, which is why common simulation tools for production processes are included in the approach. This allows economical, ecological, safety-relevant and technical aspects to be integrated into the planning process. The proposed concept is further discussed and planned for validation on the basis of a demonstrator currently under construction.<br

How Intermodal Interaction Affects the Performance of Deep Multimodal Fusion for Mixed-Type Time Series - Dietz, Simon; Altstidl, Thomas Robert; Zanca, Dario; Eskofier, Björn; Nguyen, An (International Joint Conference on Neural Networks (IJCNN), 2024)
Mixed-type time series (MTTS) is a bimodal data type that is common in many domains, such as healthcare, finance, environmental monitoring, and social media. It consists of regularly sampled continuous time series and irregularly sampled categorical event sequences. The integration of both modalities through multimodal fusion is a promising approach for processing MTTS. However, the question of how to effectively fuse both modalities remains open. In this paper, we present a comprehensive evaluation of several deep multimodal fusion approaches for MTTS forecasting. Our comparison includes different fusion types (early, intermediate, and late) and fusion methods (concatenation, weighted mean, weighted mean with correlation, gating, and feature sharing). We evaluate these fusion approaches on three distinct datasets, one of which was generated using a novel framework. This framework allows for the control of key data properties, such as the strength and direction of intermodal interactions, modality imbalance, and the degree of randomness in each modality, providing a more controlled environment for testing fusion approaches. Our findings show that the performance of different fusion approaches can be substantially influenced by the direction and strength of intermodal interactions. The study reveals that early and intermediate fusion approaches excel at capturing fine-grained and coarse-grained cross-modal features, respectively. These findings underscore the crucial role of intermodal interactions in determining the most effective fusion strategy for MTTS forecasting.<br
Keywords: mixed-type; time series; multimodal; fusion; synthetic data

Influence of hard encapsulation onto reliability of soldered die-attach in power modules - Sprenger, Mario; Forndran, Freerik; Öztürk, Beyza; Sippel, Marcel; Ockel, Manuela; Goth, Christian; Franke, Jörg (ETG-Fachbericht, 2024)
The standard approach for encapsulation of power modules is potting with soft silicone gels, a viable alternative is the usage of epoxy-based resins either applied by potting or large-area transfer molding. Within the study to be presented, standard frame-based automotive power modules with sixpack topology, soldered bottom-side and Al-wire bonded top-side die-attach are encapsulated with standard soft silicone gels as well as different epoxy-based resins, allowing for direct assessment of encapsulants reliability influence. Lifetime tests by means of active power cycling according to automotive qualification guideline AQG324 have been carried out, failure modes and failure characteristics are subsequently assessed via scanning acoustic microscopy and optical microscopy on cross sections. Focus has thereby been set onto the soldered die-attach. Experimental results show, that for the evaluated power module package the usage of epoxy-based encapsulants, which are mechanically stiffer compared to silicone gel, leads to stabilization of the chip solder layer, completely eliminating failures due to solder fatigue. Hard encapsulation shifts the failure location in power cycling for the assessed module setup completely to the top-side die attach. Assessment of different epoxy-based encapsulation materials has shown that the endured number of power cycles is directly related to their thermo-mechanical stiffness. Examinations do show that full potential exploitation of reliability-increase by fatigue reduction of soldered and/or planar interconnects by hard encapsulation is only possible with altered top-side die attach, such as soldered Cu-clips.
CRIS-Team Scopus Importer:2024-09-06

Influence of Laser Beam Intensity Distribution on Keyhole Geometry and Process Stability Using Green Laser Radiation - Kaufmann, Florian; Maier, Andreas; Schrauder, Julian; Roth, Stephan; Schmidt, Michael (Lecture Notes in Mechanical Engineering, 2024)
Laser beam welding is being more frequently employed to join copper materials. The use of green laser radiation shows the advantage of significantly higher absorptivity for these metals compared to near-infrared radiation. Therefore, a change in process stability and defect formation is expected. In addition, the effect of modifying the intensity distribution on the formation of weld seam defects and the geometric properties of the seam in deep penetration mode is largely unexplored. Thus, the aim of this work is the characterization of process dynamics and defect formation in correlation to the focal position and the intensity distribution by means of high-speed imaging and metallographic analysis. A significant reduction of seam imperfections is observed for a gaussian beam profile compared to a Top Hat intensity distribution. An advantageous seam shape and the earlier onset of the deep penetration welding process are favorable reasons for the application of this intensity distribution, while medium to high processing speeds further improve the processing quality.
CRIS-Team Scopus Importer:2023-09-29
Keywords: electromobility; green laser radiation; intensity distribution; Laser beam welding; process observation; quality improvement

Influence of molecular hydrogen on bulk and interfacial properties of three imidazolium-based ionic liquids by experiments and molecular dynamics simulations - Zhai, Ziwen; Hantal, György; Cherian, Arsha; Bergen, Alexander; Chu, Junyu; Wick, Christian; Meyer, Karsten; Smith, Ana-Suncana; Koller, Thomas Manfred (International Journal of Hydrogen Energy, 2024)
Despite the presence of molecular hydrogen (H2) in various technical applications involving ionic liquids (ILs), its effect on the bulk and interfacial properties of ILs is poorly understood. The present study investigates the influence of dissolved H2 on the viscosity, surface tension, and phase composition of three imidazolium-based ILs by experiments and Molecular Dynamics (MD) simulations between (303 and 393) K from (0.1 to 31) MPa. The surface light scattering and pendant-drop experiments showed for all three ILs that the saturated liquid viscosity does not significantly change with increasing H2 pressure, while the surface tension decreases about 5% at 8 MPa. The MD simulations could be used to clarify microscopic origins for the behavior of the macroscopic properties. They revealed not only a compensation of compression and solvation effects due to hydrostatic pressure and dissolved H2 on the viscosity, but also a weak enrichment of H2 at the IL-gas interfaces.
CRIS-Team Scopus Importer:2024-06-14
Keywords: Experiments; Hydrogen; Ionic liquids; Molecular dynamics simulations; Surface tension; Viscosity

Interactions of the Ionic Liquid [C2C1Im][DCA] with Au(111) Electrodes: Interplay between Ion Adsorption, Electrode Structure, and Stability - Yang, Juntao; Hilpert, Felix; Qiu, Yunsheng; Franz, Evanie; Briega-Martos, Valentín; Cherevko, Serhiy; Mayrhofer, Karl; Brummel, Olaf; Libuda, Jörg (Journal of Physical Chemistry C, 2024)
Modification of electrocatalysts with ionic liquids (ILs) allows us to control the activity and selectivity on the one hand but also directly affects the stability of the electrode. In this work, we studied the interaction of the IL 1-ethyl-3-methylimidazolium dicyanamide [C2C1Im][DCA] with a Au(111) surface in an aqueous electrolyte (phosphate buffer, pH 7) and the impact of the IL on the electrochemical stability of the surface. We combined complementary electrochemical in situ methods, namely, electrochemical infrared reflection absorption spectroscopy (EC-IRRAS), cyclic voltammetry (CV), electrochemical scanning tunneling microscopy (EC-STM), and online inductively coupled plasma mass spectrometry. We observe the potential-dependent and reversible adsorption-desorption of ionic species on the Au(111) surface in the potential window between 0.1 and 1.1 VRHE. We identified two types of adsorbed [DCA]− species, showing different adsorption kinetics. Already at low concentrations, the IL (10 mM) enhances the dissolution during oxidation and reduction cycles by a factor of more than 40. We observe a suppressed redeposition of dissolved Au-ions in the presence of the IL due to the formation of gold complexes with the [DCA]− anions. Our result provides molecular insights into the interplay between ionic liquid-surface interactions and surface stability.
CRIS-Team Scopus Importer:2024-03-08

Intramolecular Triplet Diffusion Facilitates Triplet Dissociation in a Pentacene Hexamer - Greißel, Phillip; Thiel, Dominik; Gotfredsen, Henrik; Chen, Lan; Krug, Marcel; Papadopoulos, Ilias; Miskolzie, Mark; Torres, Tomás; Clark, Timothy; Brøndsted Nielsen, Mogens; Tykwinski, Rik R.; Guldi, Dirk Michael (Angewandte Chemie International Edition, 2024)
Triplet dynamics in singlet fission depend strongly on the strength of the electronic coupling. Covalent systems in solution offer precise control over such couplings. Nonetheless, efficient free triplet generation remains elusive in most systems, as the intermediate triplet pair 1(T1T1) is prone to triplet-triplet annihilation due to its spatial confinement. In the solid state, entropically driven triplet diffusion assists in the spatial separation of triplets, resulting in higher yields of free triplets. Control over electronic coupling in the solid state is, however, challenging given its sensitivity to molecular packing. We have thus developed a hexameric system (HexPnc) to enable solid-state-like triplet diffusion at the molecular scale. This system is realized by covalently tethering three pentacene dimers to a central subphthalocyanine scaffold. Transient absorption spectroscopy, complemented by theoretical structural optimizations and steady-state spectroscopy, reveals that triplet diffusion is indeed facilitated due to intramolecular cluster formation. The yield of free triplets in HexPnc is increased by a factor of up to 14 compared to the corresponding dimeric reference (DiPnc). Thus, HexPnc establishes crucial design aspects for achieving efficient triplet dissociation in strongly coupled systems by providing avenues for diffusive separation of 1(T1T1), while, concomitantly, retaining strong interchromophore coupling which preserves rapid formation of 1(T1T1).
CRIS-Team Scopus Importer:2024-02-23
Keywords: Oligoacene; Photoenergy Conversion; Singlet Fission; Triplet Dissociation; Ultrafast Spectroscopy

Investigating Laser Direct Structuring and Aerosol-Jet for Additively Manufactured Helix Antennas - Ankenbrand, Markus; Panusch, David; Piechulek, Niklas; Gold, Gerald; Helmreich, Klaus; Franke, Jörg (2024 International Conference on Electronics Packaging, ICEP 2024, 2024)
This paper investigates the manufacturing of helix antennas using additive technologies, highlighting the influences of substrate surface quality and manufacturing processes on antenna characteristics. The focus lies on Fused Filament Fabrication and Digital Light Synthesis for the substrate generation, and Aerosol-Jet and Laser Direct Structuring (LDS) for functional structuring. The tested materials are PEEK, epoxy and polycarbonate for the substrate and silver and copper for the traces. Challenges encountered during manufacturing include spreading of the ink in Aerosol-Jet printing, and short circuits caused by smoke residues in LDS. The radiation patterns of the additively manufactured antennas are compared with simulated responses and a subtractively manufactured reference, revealing a small but measurable impact of geometric deviations and surface roughness on performance.
CRIS-Team Scopus Importer:2024-06-21
Keywords: Additive Manufacturing; Helix Antenna; Laser Direct Structuring; Printed Electronics

Investigation of a Decentralized Energy Management System for Undersupplied EV Charging Parks - Schwanninger, Raffael; Friedrich, Juliane; Lavery, Melanie; März, Martin (2024 IEEE Sixth International Conference on DC Microgrids (ICDCM), 2024)
In this paper, an energy management system (EMS) for undersupplied EV charging parks is presented. This undersupply is due to the maximum power demand for all charging stations connected to a common DC grid being larger than the maximum in feed power from AC mains. The EMS is completely decentralized and therefore allows easy addition of additional participants like chargers, AC/DC converters or storage systems into the common DC grid. This decentralization requires changes to the control structure of the connected power converters. These changes are evaluated for their effect on the stability of the whole grid. To investigate the proposed EMS, a simulation model based on real world charging park data is set up. The model is then used to evaluate the EMS for different degrees of undersupply and buffer storage. The findings are evaluated for different performance indicators to show the trade off between peak power shaving, installed storage and the charging of EV

Investigation of the degradation of proton exchange membrane water electrolysis cells using electrochemical impedance spectroscopy with distribution of relaxation times analysis - Sánchez Batalla, B.; Bachmann, Julien; Weidlich, C. (Electrochimica Acta, 2024)
Reduction of catalyst loading on Proton Exchange Membrane (PEM) Electrolysis Cells (EC) is needed because of scarcity and high price of the catalyst. Monitoring the degradation of these cells is important to be able to allocate the different processes taking place. Electrochemical Impedance Spectroscopy (EIS) might be suitable for this purpose, but in its usual implementation, the system and the contribution of its components to the total resistance must be already known to build a suitable equivalent circuit model (ECM). Furthermore, the overlap of different processes in the Nyquist and Bode plots hinders the identification of single components of the system. The Distribution of Relaxation Times (DRT) analysis converts the EIS data into a distribution of time constants of individual processes. The contribution of each component to the full cell resistance can be identified by varying different operation parameters. In this work, a PEMEC with low iridium loading was analyzed by DRT and the faradaic processes were identified. A long-term test was carried out and the degradation of the cell was investigated by DRT analysis, to determine the components and processes which limit the cell performance.
CRIS-Team Scopus Importer:2023-12-01
Keywords: Degradation; Distribution of relaxation times; Electrochemical impedance spectroscopy; Long-term performance; Proton exchange membrane water electrolysis cells

Investigations of selected influencing process factors for transient liquid phase soldering (TLPS) as die-attach method for automotive power modules - Ottinger, Bettina; Holverscheid, Joshua; König, Sebastian; Brunner, Annette; Müller, Lars; Goth, Christian; Franke, Jörg (International Conference and Exhibition on High Temperature Electronics (HITEC 2023), 2023)
Increasing the operating temperatures in power electronic applications cause a decreasing lifetime of the die-attach interconnection. Transient liquid phase soldering (TLPS) is able to create a reliable and high temperature resistant connection layer via diffusion processes in the form of intermetallic phases with melting points up to > 400°C. This study concentrates on a TLPS reflow-soldering process with a tin-copper-tin (Sn-Cu-Sn) layered preform. During the soldering process the influence of the selected process parameters for the Sn-Cu-Sn on the grown intermetallic phase interconnections between the preform and the components are investigated via the average void percentage and a process capability analysis (cpK). The application of mechanical pressure decreases the average void percentage below 10%. Due to the lower average void percentage the influence of the holding time, peak temperature and heating gradient are examined under low pressure. The biggest impact shows the heating gradient. With a slow heating gradient, the thermomechanical stress decreases by increasing the solubility and the average void percentage could be reduced to less than 2.7 %. In combination of the optimized selected process factors it is possible to reach a process capability value of
Keywords: die-attach; TLPS; Sn-Cu-Sn; thin solder layers; reflow-soldering process; automotive power module

Investigations Selected Influencing of Process Factors for Transient Liquid Phase Soldering (TLPS) as a Die-attach Method for Automotive Power Modules - Ottinger, Bettina; Holverscheid, Joshua; König, Sebastian; Brunner, Annette; Müller, Lars; Goth, Christian; Franke, Jörg (Advancing Microelectronics, 2024)
Increasing the operating temperatures in power electronic applications cause a decreasing lifetime of the die-attach interconnection. Transient liquid phase soldering (TLPS) is able to create a reliable and high temperature resistant connection layer via diffusion processes in the form of intermetallic phases with melting points up to > 400°C. This study concentrates on a TLPS reflow-soldering process with a tin-copper-tin (Sn-Cu-Sn) layered preform. During the soldering process the influence of the selected process parameters for the Sn-Cu-Sn on the grown intermetallic phase interconnections between the preform and the components are investigated via the average void percentage and a process capability analysis (cpK). The application of mechanical pressure decreases the average void percentage below 10%. Due to the lower average void percentage the influence of the holding time, peak temperature and heating gradient are examined under low pressure. The biggest impact shows the heating gradient. With a slow heating gradient, the thermomechanical stress decreases by increasing the solubility and the average void percentage could be reduced to less than 2.7 %. In combination of the optimized selected process factors it is possible to reach a process capability value of 1.
CRIS-Team Scopus Importer:2024-05-31
Keywords: automotive power module; die-attach; reflow-soldering process; Sn-Cu-Sn; thin solder layers; TLPS

Low-temperature performance of GeSn-on-Si avalanche photodiodes toward single-photon detection - Wanitzek, Maurice; Hack, Michael; Schwarz, Daniel; Schulze, Jörg; Oehme, Michael (Materials Science in Semiconductor Processing, 2024)
Avalanche photodiodes utilizing GeSn as an absorbing material can be used for low-light detection of telecommunication wavelengths, such as 1550 nm. We present the performance of GeSn-on-Si avalanche photodiodes at low-temperatures down to 110 K. It was found, that at temperatures above 190 K the dark current is dominated by generation-recombination due to trap states at the SiO2-semiconductor interface with an activation energy of ∼0.2 eV. At lower temperatures and high reverse voltages the leakage mechanism shifts to trap-assisted-tunneling. Responsivity measurements show the shift of the absorption edge toward shorter wavelengths. For 1550 nm and at 140 K a responsivity of 0.097 A/W is achieved, which is five times higher than a Ge reference-APD. When biased in the Geiger-mode, measurements of the dark count rates were performed, marking the first step towards GeSn-on-Si single-photon detectors for 1550 nm radiation.
CRIS-Team Scopus Importer:2024-03-15
Keywords: Avalanche photodiode; Dark current; Geiger-mode; Germanium–tin; Infrared; Quantum application

Low Ti Additions to Stabilize Ru-Ir Electrocatalysts for the Oxygen Evolution Reaction - Lahn, Leopold; Mingers, Andrea M.; Savan, Alan; Ludwig, Alfred; Kasian, Olga (ChemElectroChem, 2023)
Anodic oxygen evolution reaction (OER) challenges large scale application of proton exchange membrane water electrolyzers (PEMWE) due to sluggish kinetics, high overpotential and extremely corrosive environment. While Ir oxides currently provide the best balance between activity and stability, the scarcity of Ir and corresponding high market price lead to poor cost-benefit factors. Mixing Ir with more stable non-precious Ti reduces the noble metal loading and may implicate stabilization, while addition of more catalytically active Ru ensures a high reaction rate. Here, we examine the activity-stability behavior of Ru-Ir-Ti thin film material libraries with low Ti-content under the OER conditions. The high sensitivity to the dissolution of the individual alloy components was achieved by using online and off-line inductively coupled plasma mass spectrometry (ICP-MS) analysis. Our data reveal that even low Ti additions improve the stability of Ru-Ir catalysts without sacrificing activity. In particular, 5 at. % of Ti enable stability increase of Ir in the Ru-Ir catalyst by a factor of 3. Moreover, this catalyst exhibits higher activity compared to the Ti-free Ru-Ir alloys with similar Ir content. Observed activity-stability trends are discussed in light of X-ray photoelectron spectroscopy data.
Created from Fastlane, Scopus look-up
Keywords: catalyst stability; online ICP-MS; oxygen evolution reaction; Ru-Ir-Ti alloy; thin film catalyst

Modified Approach for the Rainflow Counting Analysis of Temperature Load Signals in Power Electronics Modules - Letz, Sebastian; März, Martin; Zhao, Dawei (13th Int. Conference on Integrated Power Electronics Systems (CIPS), 2024)

Monolithic Two-Terminal Tandem Solar Cells Using Sb2S3 and Solution-Processed PbS Quantum Dots Achieving an Open-Circuit Potential beyond 1.1 V - Kern, Selina; Yi, Gyusang; Büttner, Pascal; Scheler, Florian; Tran, Minh Hoa; Korenko, Sofia; Dehm, Katharina; Kundrata, Ivan; Zahl, Achim; Albrecht, Steve; Bachmann, Julien; Crisp, Ryan (ACS Applied Materials and Interfaces, 2024)
Multijunction solar cells have the prospect of a greater theoretical efficiency limit than single-junction solar cells by minimizing the transmissive and thermalization losses a single absorber material has. In solar cell applications, Sb2S3 is considered an attractive absorber due to its elemental abundance, stability, and high absorption coefficient in the visible range of the solar spectrum, yet with a band gap of 1.7 eV, it is transmissive for near-IR and IR photons. Using it as the top cell (the cell where light is first incident) in a two-terminal tandem architecture in combination with a bottom cell (the cell where light arrives second) of PbS quantum dots (QDs), which have an adjustable band gap suitable for absorbing longer wavelengths, is a promising approach to harvest the solar spectrum more effectively. In this work, these two subcells are monolithically fabricated and connected in series by a poly(3,4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS)-ZnO tunnel junction as the recombination layer. We explore the surface morphology of ZnO QD films with different spin-coating conditions, which serve as the PbS QD cell’s electron transport material. Furthermore, we examine the differences in photogenerated current upon varying the PbS QD absorber layer thickness and the electrical and optical characteristics of the tandem with respect to the stand-alone reference cells. This tandem architecture demonstrates an extended spectral response into the IR with an open-circuit potential exceeding 1.1 V and a power conversion efficiency of 5.6%, which is greater than that of each single-junction cell.
CRIS-Team Scopus Importer:2024-03-29
Keywords: antimony sulfide; atomic layer deposition; chalcogenide solar cells; lead sulfide; mapping ellipsometry; multijunction; nanocrystals; recombination layer

Novel Device for Fast Detection and Limitation of Short-Circuit Currents in LVDC Grids - Gehring, Johannes; Wunder, Bernd; Schwanninger, Raffael; März, Martin; Lorentz, Vincent R. H. (2024 IEEE Sixth International Conference on DC Microgrids (ICDCM), 2024)
This paper introduces a novel non-destructive device for detecting and limiting short-circuit currents in LVDC grids. The proposed solution is based on the concept of using a transformer fully saturated by a permanent magnet during normal operation. When a short circuit condition occurs, the transformer desaturates, triggering the detection and protection mechanism. The function and structure of the unit as well as design criteria are explained. Experimental results demonstrate the effectiveness of the device in detecting and limiting overcurrents. This hardware-based detection solution is particularly suitable for being installed in a semiconductor circuit breaker rated up to but not limited to 100
Keywords: Limiting; Short-circuit currents; Circuit breakers; Microgrids; Permanent magnets; Circuit faults; Protection

Observation of the local electromechanical response in 2-2 ceramic-ceramic lead-free ferroelectric composites via digital image correlation - Martin, Alexander; Maier, Juliana; Kakimoto, Ken Ichi; Kamlah, Marc; Webber, Kyle Grant (Journal of Applied Physics, 2024)
This study investigates bilayers of 0.94(Na1/2Bi1/2)TiO3-0.06BaTiO3 (NBT-6BT) and 0.90(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.04(K0.5Na0.5)NbO3 (NBT-6BT-4KNN) using digital image correlation, enabling the separate analysis of strain response in each layer. The bilayers were electrically connected without mechanical connection (polarization coupled) as well as mechanically and electrically connected (polarization and strain coupled) to determine the role of interlayer mechanical interactions. The large signal longitudinal and transverse piezoelectric coefficients, d 33 ∗ and d 31 ∗ , were characterized for both cases. In the polarization coupled composite, d 33 ∗ decreased linearly from 410 to 260 pm/V with increasing vol. % NBT-6BT. In contrast, in the polarization and strain coupled case, d 33 ∗ and d 31 ∗ were maximum at 50 vol. % NBT-6BT with values of 440 and −130 pm/V, respectively, highlighting the critical role of strain interactions in ceramic-ceramic composites. The stress-induced phase transformation through strain coupling significantly impacted the electromechanical response, with improved strain values observed in the NBT-6BT-4KNN layer. Furthermore, this study highlights the variability in the significance of strain coupling within bilayer systems as a function of the altering volume fraction of their constituent components.
CRIS-Team Scopus Importer:2024-02-16

Optimierung der Prozessparameter beim Selektivwellenlöten mit Hilfe von Offline Reinforcement Learning - Rachinger, Ben; Seidel, Reinhardt; Meier, Sven; Franke, Jörg (GMM-Fb. 107: EBL 2024 – Elektronische Baugruppen und Leiterplatten, 2024)
Im Zuge der Miniaturisierung elektrischer Baugruppen wurden THT-Komponenten weitgehend durch SMT-Bauteile verdrängt. Dennoch verbleiben zumeist vereinzelte Bauteile in Durchsteckmontage aufgrund deren überlegener mechanischer und thermischer Widerstandsfähigkeit [1], [2]. Für diese ausgewählten THT-Komponenten stellt das Selektivwellenlöten ein attraktives Verbindungsverfahren dar, weil dieses mit einem minimalen thermischen Einfluss auf umgebende SMT-Bauteile einhergeht [3]. Das Erreichen eines hohen Lotdurchstiegs ist bei diesem Verfahren stark von der Lötstellengeometrie und den gewählten Prozessparametern abhängig [4]. Bei der Einführung neuer Leiterplattendesigns in die Serienproduktion werden diese Parameter jedoch aktuell häufig durch aufwändige Versuchsreihen ausgewählt [5]. Ziel des Beitrags ist es, diesen Prozess durch eine objektive Optimierung der Prozessparameter mittels Reinforcement Learning (RL) auf Basis vorhandener Prozessdaten zu beschleunigen. Neben Gerberdaten zur Beschreibung der Lötstellengeometrie werden Bauteilcharakteristika und initiale Prozessparameter verwendet. RL ermöglicht eine gleichzeitige Modellierung und Optimierung des Prozesses, sodass Parameter empfohlen werden, welche das Erzielen eines hohen Lotdurchstiegs ermöglichen. Während der Anwendungsphase erlaubt das Framework über eine grafische Benutzeroberfläche eine Interaktion zwischen dem RL-Modell und dem Prozessexperten. Diese können gemeinsam basierend auf Trainingsdaten sowie Expertenwissen die Parameter schrittweise und zielgerichtet optimieren. Eine Validierung des Frameworks mittels Lötversuchen zeigt, dass bereits vor Beginn dieser Interaktion die empfohlenen Parameter für 75 % der getesteten Verbindungen einen Lotdurchstieg gemäß IPC-A-610 Klasse 3 [6] erzeuge

Optimizing Intralogistics in an Engineer-to-Order Enterprise with Job Shop Production: A Case Study of the Control Cabinet Manufacturing - Bründl, Patrick; Stoidner, Micha; Nguyen, Huong Giang; Abrass, Ahmad; Franke, Jörg (SAE International Journal of Materials and Manufacturing, 2024)
This study underscores the benefits of refining the intralogistics process for small- to medium-sized manufacturing businesses (SMEs) in the engineer-to-order (ETO) sector, which relies heavily on manual tasks. Based on industrial visits and primary data from six SMEs, a new intralogistics concept and process was formulated. This approach enhances the value-added time of manufacturing workers while also facilitating complete digital integration as well as improving transparency and traceability. A practical application of this method in a company lead to cutting its lead time by roughly 11.3%. Additionally, improved oversight pinpointed excess inventory, resulting in advantages such as reduced capital needs and storage requirements. Anticipated future enhancements include better efficiency from more experienced warehouse staff and streamlined picking methods. Further, digital advancements hold promise for cost reductions in administrative and supportive roles.<br
Keywords: Aerospace; Automotive; Commercial Vehicle; Research and development; Production; Logistics; Traceability; Storage; Materials handling; Cutting; Optimization; Personnel; Supply chain management

Perceived algorithmic fairness: An empirical study of transparency and anthropomorphism in algorithmic recruiting - Ochmann, Jessica; Michels, Leonard; Tiefenbeck, Verena; Maier, Christian; Laumer, Sven (Information Systems Journal, 2024)
Despite constant efforts of organisations to ensure a fair and transparent personnel selection process, hiring is still characterised by systematic inequality. The potential of algorithms to produce fair and objective decision outcomes has attracted the attention of academic scholars and practitioners as a conceivable alternative to human decision-making. However, applicants do not necessarily consider an objective algorithm as fairer than a human decision maker. This study examines the conditions under which applicants perceive algorithms as fair and establishes a theoretical foundation of algorithmic fairness perceptions. We further propose and investigate transparency and anthropomorphism interventions as strategies to actively shape these fairness perceptions. In an online application scenario with eight experimental groups (N = 801), we analyse determinants for algorithmic fairness perceptions and the impact of the proposed interventions. Embedded in a stimulus-organism-response framework and drawing from organisational justice theory, our study reveals four justice dimensions (procedural, distributive, interpersonal, informational justice) that determine algorithmic fairness perceptions. The results further show that transparency and anthropomorphism interventions mainly affect dimensions of interpersonal and informational justice, highlighting the importance of algorithmic fairness perceptions as critical determinants for individual choice
Keywords: Diversity

Power Forecast of Photovoltaic Systems-An Approach for Improving Energy Management of DC-Supplied Production Plants - Gutwald, Benjamin; Barth, Martin; Mönius, Oliver; Zeilmann, B.; Franke, Jörg (Congress of the German Academic Association for Production Technology, 2024)
Regeneratively fed Direct Current (DC) grids play a key role in the energy transition. The promising supply architecture for industrial plants is not only materially efficient in its topology, but it also distributes energy without large conversion losses, especially for electric drives. While the architecture already offers a particularly efficient integration for renewable energies, the natural fluctuation must be mastered in energy management. Depending on the orientation, dimensioning and geographical conditions, this paper describes an approach and implementation of power forecasting for photovoltaic (PV) systems based on numerical weather predictions. Furthermore, the associated energy management scenarios for storage and loads in DC-supplied production plants are presented. The advantages of energy forecasting lie in the optimal design of the DC-Grid components, the improvement of peak shaving and the early energy optimisation of production control through connection to a Manufacturing Execution System (MES).
CRIS-Team Scopus Importer:2023-12-15
Keywords: energy management; energy optimised manufacturing execution system; photovoltaic forecast for grid serving operation

Preparation of atom probe tips from (nano)particles in dispersion using (di)electrophoresis and electroplating - Vorlaufer, Nora; Josten, Jan; Carl, Simon; Göbel, Erik; Soegaard, Alexander; Taccardi, Nicola; Spiecker, Erdmann; Felfer, Peter (Microscopy Research and Technique, 2024)
The behavior of catalytic particles depends on their chemical structure and morphology. To reveal this information, the characterization with atom probe tomography has huge potential. Despite progresses and papers proposing various approaches towards the incorporation of particles inside atom probe tips, no single approach has been broadly applicable to date. In this paper, we introduce a workflow that allowed us to prepare atom probe specimens from Ga particles in suspension in the size range of 50 nm up to 2 μm. By combining dielectrophoresis and electrodeposition in a suitable way, we achieve a near-tip shape geometry, without a time-consuming FIB lift-out. This workflow is a simple and quick method to prepare atom probe tips and allows for a high preparation throughput. Also, not using a lift-out allowed us to use a cryo-stage, avoiding melting of the Ga particles, while ensuring a mechanical stable atom probe tip. The specimen prepared by this workflow enable a stable measurement and low fracture rates. Research Highlights: Enabling cryo-preparation of (nano)particles for the atom probe. Characterization of surface and bulk elemental distribution of GaPt model SCALMS.
CRIS-Team Scopus Importer:2024-02-16
Keywords: atom probe tomography; catalyst particles; cryo preparation; nanoparticles

Pt Single Atoms Loaded on Thin-Layer TiO2 Electrodes: Electrochemical and Photocatalytic Features - Zhou, Xin; Wang, Yue; Denisov, Nikita; Kim, Hyesung; Kim, Ji Hyeon; Will, Johannes; Spiecker, Erdmann; Vaskevich, Alexander; Schmuki, Patrik (Small, 2024)
Recently, the use of Pt in the form of single atoms (SA) has attracted considerable attention to promote the cathodic hydrogen production reaction from water in electrochemical or photocatalytic settings. First, produce suitable electrodes by Pt SA deposition on Direct current (DC)-sputter deposited titania (TiO2) layers on graphene—these electrodes allow to characterization of the electrochemical properties of Pt single atoms and their investigation in high-resolution HAADF-STEM. For Pt SAs loaded on TiO2, electrochemical H2 evolution shows only a very small overpotential. Concurrent with the onset of H2 evolution, agglomeration of the Pt SAs to clusters or nanoparticles (NPs) occurs. Potential cycling can be used to control SA agglomeration to variable-size NPs. The electrochemical activity of the electrode is directly related to the SA surface density (up to reaching the activity level of a plain Pt sheet). In contrast, for photocatalytic H2 generation already a minimum SA density is sufficient to reach control by photogenerated charge carriers. In electrochemical and photocatalytic approaches a typical TOF of ≈100–150 H2 molecules per second per site can be reached. Overall, the work illustrates a straightforward approach for reliable electrochemical and photoelectrochemical investigations of SAs and discusses the extraction of critical electrochemical factors of Pt SAs on titania electrodes.
CRIS-Team Scopus Importer:2024-08-23
Keywords: anatase thin film electrode; electrocatalytic hydrogen evolution; graphene; photocatalytic hydrogen evolution; Pt Single atom

Quality Control in Particle Precipitation via Robust Optimization - Kuchlbauer, Martina; Dienstbier, Jana; Muneer, Adeel; Hedges, Hanna; Stingl, Michael; Liers, Frauke; Pflug, Lukas (Computers & Chemical Engineering, 2024)

Quick Large-Area Detection of Thin Silicone Films with Coherent Raman Scattering Imaging - Naser, Julian; Sarau, George; Wrege, Jan; Schmidt, Michael; Christiansen, Silke (physica status solidi (a), 2024)
Coherent Raman Scattering Imaging offers quick and surface-sensitive large-area scans of specimens including fingerprinting of the substances, combined with low detection limits. By using a multi-photon process, it offers orders of magnitude stronger surface signal in comparison to spontaneous Raman (SR) and fluorescence signals of the substrate are avoided by the detection principle and the near-infrared laser wavelength. By exciting at a specific wavelength instead of measuring whole spectra the measurement time is remarkably reduced. Here, detecting organic thin films on glass surfaces is investigated. The detection of those thin films is relevant for production processes where the interfacial layer is influenced by the low surface energy of contaminants, especially polysiloxanes. The samples are manufactured by pipetting diluted polydimethylsiloxane (PDMS) in heptane in various concentrations (1%, 0.1%, 0.01%, and 0.001%) on glass substrates. After evaporation of the solvent, thin films of the silicone remain. The resulting average coverage of the thin films equals 100–0.1 μg cm−2. Measurements determine the detectability threshold with SR to 10 μg cm−2, while the acquired spectrum is used to determine characteristic peaks for the pump laser. Afterwards the samples are imaged at those wavenumbers with Coherent Raman Scattering (CRS) Imaging techniques providing a chemically-specific contrast.
CRIS-Team Scopus Importer:2024-03-15
Keywords: cars; contamination monitoring; raman; silicone; srs; thin film detection

Reactivity of a model SCILL: Influence of co-adsorbed [C2C1Im][OTf] on the dehydrogenation of dimethylamine on Pt(111) - Eschenbacher, Roman; Steffen, Julien; Farrugia, Karl; Taccardi, Nicola; Wasserscheid, Peter; Görling, Andreas; Libuda, Jörg (Surface Science, 2024)
Solid catalysts with ionic liquid layers (SCILLs) are heterogeneous catalysts whose selectivity is significantly improved by the application of a thin coating of an ionic liquid (IL). In this work, we studied the influence of the IL 1-ethyl-3-methyl-imidazolium-trifluormethansulfonate ([C2C1Im][OTf]) on the dehydrogenation of dimethylamine (DMA) on Pt(111) by surface science experiments under ultrahigh vacuum (UHV) conditions. We investigated the growth, thermal stability, and the dehydrogenation reaction of DMA on Pt(111) both in the presence and absence of [C2C1Im][OTf] by time-resolved and temperature-programmed infrared reflection absorption spectroscopy (IRAS) and density functional theory (DFT). We found that pre-adsorbed DMA has a strong influence on the interaction of [C2C1Im][OTf] with the Pt surface. The competition for the same adsorption sites leads to changes in the binding motif and binding angles of the IL. In the presence of the IL, the reaction temperature of DMA is shifted to higher temperature due to the same competition and blocking effects. Thus, the IL leads to an enhanced stability of DMA on the surface and influences the product spectrum of the dehydrogenation reaction at higher temperature.
CRIS-Team Scopus Importer:2024-03-01
Keywords: Catalysis; Dehydrogenation; Infrared spectroscopy; Ionic liquid; SCILL

Real-World Gait Detection Using a Wrist-Worn Inertial Sensor: Validation Study - Kluge, Felix; Brand, Yonatan E.; Micó-Amigo, M. Encarna; Bertuletti, Stefano; D'Ascanio, Ilaria; Gazit, Eran; Bonci, Tecla; Kirk, Cameron; Küderle, Arne; Palmerini, Luca; Paraschiv-Ionescu, Anisoara; Salis, Francesca; et al. (JMIR Formative Research, 2024)
Background: Wrist-worn inertial sensors are used in digital health for evaluating mobility in real-world environments. Preceding the estimation of spatiotemporal gait parameters within long-term recordings, gait detection is an important step to identify regions of interest where gait occurs, which requires robust algorithms due to the complexity of arm movements. While algorithms exist for other sensor positions, a comparative validation of algorithms applied to the wrist position on real-world data sets across different disease populations is missing. Furthermore, gait detection performance differences between the wrist and lower back position have not yet been explored but could yield valuable information regarding sensor position choice in clinical studies. Objective: The aim of this study was to validate gait sequence (GS) detection algorithms developed for the wrist position against reference data acquired in a real-world context. In addition, this study aimed to compare the performance of algorithms applied to the wrist position to those applied to lower back-worn inertial sensors. Methods: Participants with Parkinson disease, multiple sclerosis, proximal femoral fracture (hip fracture recovery), chronic obstructive pulmonary disease, and congestive heart failure and healthy older adults (N=83) were monitored for 2.5 hours in the real-world using inertial sensors on the wrist, lower back, and feet including pressure insoles and infrared distance sensors as reference. In total, 10 algorithms for wrist-based gait detection were validated against a multisensor reference system and compared to gait detection performance using lower back-worn inertial sensors. Results: The best-performing GS detection algorithm for the wrist showed a mean (per disease group) sensitivity ranging between 0.55 (SD 0.29) and 0.81 (SD 0.09) and a mean (per disease group) specificity ranging between 0.95 (SD 0.06) and 0.98 (SD 0.02). The mean relative absolute error of estimated walking time ranged between 8.9% (SD 7.1%) and 32.7% (SD 19.2%) per disease group for this algorithm as compared to the reference system. Gait detection performance from the best algorithm applied to the wrist inertial sensors was lower than for the best algorithms applied to the lower back, which yielded mean sensitivity between 0.71 (SD 0.12) and 0.91 (SD 0.04), mean specificity between 0.96 (SD 0.03) and 0.99 (SD 0.01), and a mean relative absolute error of estimated walking time between 6.3% (SD 5.4%) and 23.5% (SD 13%). Performance was lower in disease groups with major gait impairments (eg, patients recovering from hip fracture) and for patients using bilateral walking aids. Conclusions: Algorithms applied to the wrist position can detect GSs with high performance in real-world environments. Those periods of interest in real-world recordings can facilitate gait parameter extraction and allow the quantification of gait duration distribution in everyday life. Our findings allow taking informed decisions on alternative positions for gait recording in clinical studies and public health.
CRIS-Team Scopus Importer:2024-05-24
Keywords: accelerometer; digital health; digital mobility outcomes; inertial measurement unit; Mobilise-D; validation; walking; wearable sensor

Recycling Concept for Electric Vehicle Drives in the Context of Rare Earth Recovery - Ihne, Thorsten; Hahn, Roman; Wieprecht, Nico; Franke, Jörg; Kühl, Alexander (2024 1st International Conference on Production Technologies and Systems for E-Mobility (EPTS), 2024)
Neodymium Iron Boron (NdFeB) magnets provide the best magnetic properties and are therefore key components for many green technologies. This leads to steeply rising demand for the related rare earth (RE) elements, with e-mobility as central driver. The supply situation for RE magnets is characterized by the dominance of the PR China for the entire value chain causing various problems for the European industry. A potential countermeasure to reduce the dependency on imports is the implementation of a closed raw material cycle within the European Union (EU). Especially the recycling of automotive traction drives provides high potential for the recovery of RE magnets but is complicated by various technical and organizational factors. As result currently no recycling is industrialized for the recovery of RE magnets from traction drives of end-of-life (EOL) electric vehicles (EV).In this context the following paper presents a general concept for the recovery of RE magnets from automotive traction motors. The focus is set on the extraction process of the magnets from the rotor and the related thermal demagnetization step. Finally, the achieved results are evaluated regarding the transferability to a potential series process.<br
Keywords: Electric potential; Europe; Rotors; Electric vehicles; Traction motors; Recycling; Magnets; Rare earth magnets; circular economy; recycling; electromobility; traction drives; thermal demagnetization

Resonating Electrostatically Guided Electrons - Seidling, Michael; Schmidt-Kaler, Franz; Zimmermann, Robert; Simonaitis, J. W.; Keathley, P. D.; Berggren, Karl K.; Hommelhoff, Peter (Physical Review Letters, 2024)
An essential component for quantum-enhanced measurements with free electrons is an electron resonator. We report stable guiding of free electrons at 50 eV energy for up to seven round trips in a linear autoponderomotive guiding structure, which is realized with two microstructured printed circuit boards that generate the required electromagnetic fields. Free electrons are laser triggered from a sharp tungsten needle tip and coupled in at the front of the electron resonator with the help of sub-nanosecond-fast switchable electron mirrors. After a variable time delay, we open the rear electron mirror and measure the number of trapped electrons with a delay-line detector. We demonstrate, simulate, and show ways of optimizing an electron resonator in simulations, which will help enable “interaction-free” measurement setups, including multipass and quantum-Zeno effect based schemes, helping to realize the quantum electron microscope.<br

Reversible C═N Bond Formation Controls Charge-Separation in an Aza-Diarylethene Photoswitch - Sacherer, Maximilian-Tobias; Gracheva, Sofia; Maid, Harald; Placht, Christian; Hampel, Frank; Dube, Henry (Journal of the American Chemical Society, 2024)
Diarylethenes belong to the most eminent photoswitches and have been studied for many decades. They are found in virtually every field of application and have become highly valuable molecular tools for instilling light-responsiveness into materials, catalysts, biological systems, or pharmacology. In this work, we present a novel and distinct type of pyrimidine-based aza-diarylethene, which undergoes a highly unusual zwitterion-forming photoreaction. During this fully reversible process, a CN double bond is established under concomitant aromatization and thiophene-ring opening. The metastable zwitterion thus possesses a positively charged extended aromatic structure and an independent conjugated thiolate function. It can further photoisomerize between a more stable Z and a less stable E isomer, resulting in effective four-state photoswitching. Unusual for diarylethenes, the metastable isomers show negative solvatochromism and red-shifted absorption in apolar solvents. With this behavior, aza-diarylethenes effectively bridge the properties of merocyanines and diarylethenes. Thermal stability of the zwitterions can be modulated from very labile to highly stable behavior in response to pH, again in a fully reversible manner. Pyrimidine-based aza-diarylethene thus establishes a unique photoreaction mechanism for diarylethenes, allowing control of charge separation, thermal stability, and color generation in a different way than hitherto possible.
CRIS-Team Scopus Importer:2024-04-26

Rhodanine-Based Chromophores: Fast Access to Capable Photoswitches and Application in Light-Induced Apoptosis Autoren - Köttner, Laura; Wolff, Friederike; Mayer, Peter; Zanin, Esther; Dube, Henry (Journal of the American Chemical Society, 2024)
Molecular photoswitches are highly desirable in all chemistry-related areas of research. They provide effective outside control over geometric and electronic changes at the nanoscale using an easy to apply, waste-free stimulus. However, simple and effective access to such molecular tools is typically not granted, and elaborate syntheses and substitution schemes are needed in order to obtain efficient photoswitching properties. Here we present a series of rhodanine-based photoswitches that can be prepared in one simple synthetic step without requiring elaborate purification. Photoswitching is induced by UV and visible light in both switching directions, and thermal stabilities of the metastable states as well as quantum yields are very high. An additional benefit is the hydrogen-bonding capacity of the rhodanine fragment, which enables applications in supramolecular or medicinal chemistry. We further show that the known rhodanine-based inhibitor SMI-16a is a photoswitchable apoptosis inducer. The biological activity of SMI-16a can effectively be switched ON or OFF by reversible photoisomerization between the inactive E and the active Z isomer. Rhodanine-based photoswitches therefore represent an easy to access and highly valuable molecular toolbox for implementing light responsiveness to the breadth of functional molecular systems.<br

Segregation-induced strength anomalies in complex single-crystalline superalloys - Bezold, Andreas; Vollhüter, Jan; Karpstein, Nicolas; Lenz, Malte; Subramanyam, Aparna P.A.; Zenk, Christopher; Hammerschmidt, Thomas; Spiecker, Erdmann; Göken, Mathias; Neumeier, Steffen (Communications Materials, 2024)
Pushing the maximum service temperature of aircraft engines and industrial gas turbines is the major pathway to improve their energy efficiency and reduce CO2 emissions. This maximum is mostly limited by the temperature capability of key-component materials, including superalloys. In this alloy class, segregation of elements facilitates plastic deformation and is generally considered to cause softening during high-temperature deformation. Here, we show that segregation-assisted processes can also lead to strengthening and induce an anomalous increase of the yield strength. Atomic-resolution transmission electron microscopy and density functional theory calculations reveal a segregation-assisted dissociation process of dislocations at precipitate-matrix interfaces in combination with atomic-scale reordering processes. These processes lead to an inhibition of athermal deformation mechanisms and a transition to stacking fault shearing, which causes the strengthening effect. Unraveling these elementary mechanisms might guide a mechanism-based alloy design of future superalloys with enhanced high-temperature capabilities.
CRIS-Team Scopus Importer:2024-02-02

Self-Assembled Amphiphilic Molecules for Highly Efficient Photocatalytic Hydrogen Evolution from Water - Lee, Hyun-Jun; Kim, Jaekwan; Abudulimu, Abasi; Cabanillas-Gonzalez, Juan; Nandajan, Paramjyothi C.; Gierschner, Johannes; Lueer, Larry; Park, Soo Young (Journal of Physical Chemistry C, 2020)
Self-assembled molecules for outstanding hydrogen evolution rate and durability should promise practical water splitting due to the versatile visible light absorption, low production cost, and ease of control. Here, we adapted an amphiphilic molecule as a building block for efficient small molecule based self-assembled photocatalyst for hydrogen evolution from water. The self-assembled molecules with platinum cocatalyst showed outstanding performance (turnover number ∼27000) virtually comparable to the state-of-the-art metal oxide based photocatalysts with catalytic activity extending over days. Transient absorption studies in combination with quantum chemical calculations revealed that elaborate excited state engineering of the molecules resulted in such high performance of hydrogen evolution from water. This study shows that the self-assembled amphiphilic molecules could pave the way to more economical and reproducible production of hydrogen from water.
CRIS-Team Scopus Importer:2020-05-02

Sensory evaluation of axillary odour samples of younger and older adults by a trained panel - Owsienko, Diana; Schwinn, Leo; Eskofier, Björn; Kiesswetter, Eva; Loos, Helene (Flavour and Fragrance Journal, 2024)
It has been reported that a distinct ‘old person smell’ can develop with advancing age, however, this odour has not yet been sufficiently described in previous research. Sensory evaluation by a trained panel might be useful to describe alterations with age in body odour (BO). To evaluate the alterations and achieve first insights into the ‘old person smell’, this pilot study determined the odour profiles of BO samples from both a younger and an older age group with a trained panel. In addition, we aimed to assess whether the panellists can recognize the age group based on the smell of the BO samples. Eight younger (20–28 years) and eight older (80–83 years) participants sampled their BO by wearing a cotton T-shirt for one night. The samples were sensorially evaluated by a trained panel, including ratings of total intensity and pleasantness. Additionally, an age labelling task was performed as a forced-choice decision. Results revealed that the odour profiles of the BO samples were very similar for both age groups. Nevertheless, trained panellists were able to predict the age group with significantly higher accuracy (p =.042) than expected by chance (61% mean accuracy over all panellists). Furthermore, a linear support vector machine (SVM) classifier achieved an average accuracy of 69%. This finding indicates that the age of a person affects the BO, though it is not reflected in significantly distinct odour profiles.
CRIS-Team Scopus Importer:2023-12-22
Keywords: ageing; body odour; odour profile; old person smell

Simulation of Carrier Injection Efficiency in AlGaN-based UV-light-emitting Diodes - Hofmann, Gregor; Muhin, Anton; Susilo, Norman; Römer, Friedhard; Wernicke, Tim; Kneissl, Michael; Witzigmann, Bernd (IEEE Photonics Journal, 2024)
Numerical simulations of carrier transport in aluminium gallium nitride based ultraviolet light emitting diodes (UV-LED) are performed in order to understand injection efficiency for light sources in the deep ultraviolet. With our simulator, calibrated with experimental data from a 265 nm UV-LED, quantum efficiencies have been analyzed. The maximum internal quantum efficiency (IQE) of 30% consists of the product from radiative recombination efficiency (RRE) of 60% and carrier injection efficiency (CIE) of 50%. It is found that poor hole injection into the active region and a surplus of electrons limit both efficiencies, and leads to significant electron leakage into the p-side. This leakage is bias dependent, and has a minimum at maximum IQE. Further simulations show that distributed polarization doping (DPD) could improve carrier injection efficiency.
CRIS-Team Scopus Importer:2024-07-26
Keywords: Current density; Doping; Efficiency; Gallium; Gallium Nitride; LED; Light Emitting Diode; Light emitting diodes; Light Source; Mathematical models; Modeling; Radiative recombination; Semiconductor process modeling

Simulation of solid-state lasers with temperature and wavelength dependent absorption and emission - Saha, Souryadeep; Mackenzie, Jacob; Pflaum, Christoph (Proceedings Computational Optics 2024, 2024)
Simulation of diode-pumped solid-state lasers (DPSSL) and amplifiers often do not account for the temperature and spectral dependencies of the absorption and emission cross sections of the gain medium. Typically, to track the pump absorption within the crystal, an average absorption coefficient is applied via a raytracing technique. The outcome, therefore, is an approximation of the pump absorption profile that is independent of the temperature profile within the gain medium. Here an iterative algorithm involving raytracing and Finite Element Analysis (FEA) is demonstrated in the simulation of neodymium(Nd) and ytterbium(Yb) doped yttrium aluminium garnet(YAG) single crystal fiber (SCF) gain media. The algorithm calculates the local temperature, associated absorption coefficient and hence temperature-dependent pump absorption. This allows for a more accurate determination of the distributions of the calculated population inversion and temperature in the crystal. The temperature dependence of the emission spectra can then be taken into account as well, which defines the achievable gain of the amplifying media. The resulting calculations’ influence on the simulated output beam quality and gain for these active media is presented.<br
Keywords: NdYAG; Solid State Laser; Temperature Metrology; Laser Crystal; DPSSL; Finite Element Method; Raytracing

Sol-gel coatings for solar cover glass: Influence of surface structure on dust accumulation and removal - Zimmermann, Thomas; Stauch, Claudia; Bittel, Laura; Jüngling, Natalie; Muhamettursun, Mahmut; Halik, Marcus; Niessner, Jennifer; Wintzheimer, Susanne; Lyons, Alan; Löbmann, Peer; Mandel, Karl (Solar Energy, 2024)
Soiling of solar cover glass is a major cause for efficiency loss of solar photovoltaic modules. Anti-soiling coatings can be used to reduce the rate of soiling and lower cleaning costs. The efficiency of these coatings has been demonstrated in laboratory and field tests, but the mechanisms and relevant parameters are still not well understood. In this article, we present a study on the influence of surface structure of hydrophilic sol–gel coatings on their anti-soiling performance in terms of both, dust accumulation and subsequent indoor tests for dust removal by wind. The surface structure of the films originates from the addition of colloidal silica particles of different sizes to the sols used for film preparation. In the dust deposition experiments, standardized test dust and dust collected from solar installations were used. Repeated tests were conducted under controlled humidity. In the case of dust deposition, the accumulation of dust particles larger than ∼10 µm is strongly reduced, in relation to bare glass, by the anti-soiling effect of all coatings regardless of their surface structure. For the dust removal via wind, coatings that bear a smoother surface structure are cleaned more easily than coatings with a rougher surface structure. Additionally, larger and rounder dust particles are removed more easily from coatings with a rougher surface structure (structure height over ∼40 nm), while smaller and more irregularly shaped dust particles are removed more easily from coatings with a smoother surface structure.
CRIS-Team Scopus Importer:2024-01-05
Keywords: Anti-dust; Anti-soiling; Clean surfaces; PV module efficiency increase; Sol–gel coatings for photovoltaics; Structure

Stability of Bimetallic PtxRuy - From Model Surfaces to Nanoparticulate Electrocatalysts - Kormanyos, Attila; Büttner, Pascal; Bosch, Michael; Minichova, Maria; Körner, Andreas; Jenewein, Ken J.; Hutzler, Andreas; Mayrhofer, Karl J.J.; Bachmann, Julien; Cherevko, Serhiy (ACS Materials Au, 2024)
Fundamental research campaigns in electrocatalysis often involve the use of model systems, such as single crystals or magnetron-sputtered thin films (single metals or metal alloys). The downsides of these approaches are that oftentimes only a limited number of compositions are picked and tested (guided by chemical intuition) and that the validity of trends is not verified under operating conditions typically present in real devices. These together can lead to deficient conclusions, hampering the direct application of newly discovered systems in real devices. In this contribution, the stability of magnetron-sputtered bimetallic PtxRuy thin film electrocatalysts (0 at. % to 100 at. % Ru content) along with three commercially available carbon-supported counterparts (50-67 at. % Ru content) was mapped under electrocatalytic conditions in acidic electrolytes using online ICP-MS. We found several differences between the two systems in the amount of metals dissolved along with the development of the morphology and composition. While the Pt-rich PtxRuy compositions remained unchanged, 30-50 nm diameter surface pits were detected in the case of the Ru-rich sputtered thin films. Contrastingly, the surface of the carbon-supported NPs enriched in Pt accompanied by the leaching of a significant amount of Ru from the alloy structure was observed. Change in morphology was accompanied by a mass loss reaching around 1-2 wt % in the case of the sputtered samples and almost 10 wt % for the NPs. Since PtxRuy has prime importance in driving alcohol oxidation reactions, the stability of all investigated alloys was screened in the presence of isopropanol. While Pt dissolution was marginally affected by the presence of isopropanol, several times higher Ru dissolution was detected, especially in the case of the Ru-rich compositions. Our results underline that trends in terms of electrocatalytic activity and stability cannot always be transferred from model samples to systems that are closer to the ones applied in real devices.
CRIS-Team Scopus Importer:2024-05-24
Keywords: bimetallic electrocatalyst; dealloying; electrocatalyst stability; material library; online ICP-MS

Structural and Electronic Impacts of the Axial Substitution at the Phosphorus Center of C(sp3)-Bridged P-Heterotriangulenes - Harrer, Siegfried; Schühle, Johannes D.R.; Neiß, Christian; Günthert, Marina; Mittmann, Lena; Reule, Andrea; Leitner, Jonas; Rominger, Frank; Görling, Andreas; Kivala, Milan (European Journal of Organic Chemistry, 2024)
The structural and electronic properties of the dimethylmethylene-bridged P-heterotriangulene have been systematically modulated by functionalization of its phosphorus center. The optoelectronic properties of the obtained derivatives in solution were characterized by UV/Vis absorption and emission spectroscopy, cyclic voltammetry and 31P NMR spectroscopy. The solid state structures were analyzed by X-ray crystallography. The experimental results were corroborated by density functional theory calculations which revealed the crucial influence of the axial substitution on the bowl-shaped geometry of the phosphorus-centered polycyclic scaffold, which is proportional to the s-character of the P−X bond. The advantage of the bowl-shaped P-heterotriangulene scaffold was demonstrated by successful co-crystallization with fullerene C60.
CRIS-Team Scopus Importer:2024-05-10
Keywords: bowl-shaped; bridged triarylphosphines; density functional theory calculations; phosphorus heterocycles; X-ray crystallography

Supported Catalytically Active Liquid Metal Solutions: Liquid Metal Catalysis with Ternary Alloys, Enhancing Activity in Propane Dehydrogenation - Moritz, Michael; Maisel, Sven; Raman, Narayanan; Wittkämper, Haiko; Wichmann, Christoph; Grabau, Mathias; Kahraman, Deniz; Steffen, Julien; Taccardi, Nicola; Görling, Andreas; Haumann, Marco; Wasserscheid, Peter; et al. (ACS Catalysis, 2024)
The research and optimization of catalysts are instrumental in revolutionizing chemical processes and making them viable in terms of energy and resources. Supported catalytically active liquid metal solutions (SCALMS) are highly active and stable in the harsh environment of the dehydrogenation reaction of alkanes. This is attributed to their liquid and dynamic nature and their isolated catalytically active single-atom sites. SCALMS consists of a liquid metal matrix (gallium) in which a catalytically active transition metal (platinum, rhodium, palladium, or nickel) is dissolved. Binary SCALMS systems are the subject of extensive research, aiming at a better understanding for achieving optimal performance. This work uses a combined multidisciplinary approach to examine ternary systems of the active transition metal platinum with mixtures of gallium with either tin or indium. Reaction engineering and surface chemical analysis by X-ray photoelectron spectroscopy are combined with density functional theory (DFT) and machine learning force field (ML-FF) simulations. Introducing a third metal into the mixture alters the reactivity, surface composition, and concentration gradient in the liquid metal catalyst. We demonstrate that changes in catalytic reactivity correlate with changes in the surface concentration of the active transition metal platinum and of the position of the d-band center. These findings hold great promise for future research directions as they offer potential starting points for developing SCALMS systems.
CRIS-Team Scopus Importer:2024-04-19
Keywords: AIMD; dehydrogenation; DFT; ML-FF; single-atom catalysis; XPS

Synthesis and Characterization of Bola-Amphiphilic Porphyrin-Perylenebisimide Architectures - Schulze, Erik; Ritterhoff, Christian; Franz, Evanie; Tavlui, Olha; Brummel, Olaf; Meyer, Bernd; Hirsch, Andreas (Chemistry - A European Journal, 2024)
We report on the synthesis and characterization of a family of three water-soluble bola-amphiphilic zinc-porphyrin-perylenebisimide triads containing oligo carboxylic-acid capped Newkome dendrons in the periphery. Variations of the perylenebisimide (PBI) core geometry and dendron size (G1 and G2 dendrons with 3- and 9-carboxylic acid groups respectively) allow for tuning the supramolecular aggregation behavior with respect to variation of the molecular architecture. The triads show good solubility in basic aqueous media and aggregation to supramolecular assemblies. Theoretical investigations at the DFT level of theory accompanied by electrochemical measurements unravel the geometric and electronic structure of the amphiphiles. UV/Vis and fluorescence titrations with varying amounts of THF demonstrate disaggregation.
CRIS-Team Scopus Importer:2024-01-19
Keywords: Aggregation; Amphiphiles; Donor-Acceptor-Hybrids; Perylenebisimide; Porphyrinoids

Synthetic Strategies for the Selective Functionalization of Carbon Nanodots Allow Optically Communicating Suprastructures - Bartolomei, Beatrice; Sbacchi, Maria; Rosso, Cristian; Günay-Gürer, Ayse; Zdrazil, Lukas; Cadranel, Alejandro; Kralj, Slavko; Guldi, Dirk Michael; Prato, Maurizio (Angewandte Chemie International Edition, 2024)
The surface of Carbon Nanodots (CNDs) stands as a rich chemical platform, able to regulate the interactions between particles and external species. Performing selective functionalization of these nanoscale entities is of practical importance, however, it still represents a considerable challenge. In this work, we exploited the organic chemistry toolbox to install target functionalities on the CND surface, while monitoring the chemical changes on the material's outer shell through nuclear magnetic resonance spectroscopy. Following this, we investigated the use of click chemistry to covalently connect CNDs of different nature en-route towards covalent suprastructures with unprecedent molecular control. The different photophysical properties of the connected particles allowed their optical communication in the excited state. This work paves the way for the development of selective and addressable CND building blocks which can act as modular nanoscale synthons that mirror the long-established reactivity of molecular organic synthesis.
CRIS-Team Scopus Importer:2024-01-26
Keywords: Carbon Nanodots; Click Chemistry; Nanotechnology; Particle Network; Self-Assembly

Tailored laser beam shapes for welding of copper using green laser radiation - Kaufmann, Florian; Roth, Stephan; Schmidt, Michael (International Journal of Advanced Manufacturing Technology, 2024)
The rapid development of laser beam sources and adapted welding technologies in recent years lead to an increased use of laser welding techniques in automated production nowadays. Especially its precision and local energy input are key features for joining applications in electric vehicle components, where joints have to meet both mechanical and electrical requirements as current-carrying connections. However, the copper materials used are difficult to weld due to their physical properties, making a stable process with fewest seam imperfections only feasible within a limited process window. Recently available beam sources emitting visible laser radiation have proven to overcome the low absorptivity at process start, but spattering is still a prone defect significantly affecting process efficiency and quality. Literature approaches for modifying the energy input point to laser beam shaping as a method for reducing process imperfections, which, however, has not been extensively researched in copper processing using green laser radiation. Thus, this study investigates the influence of a shaped intensity profile for visible laser radiation created with a reflective diffractive optical element in laser beam welding with laser powers up to 3 kW. A characterization of the process dynamics is performed by use of high-speed imaging, and metallographic analysis is used to elaborate benefits of the applied beam shapes. With beam shaping, an enlarged heat conduction welding regime and an advantageous seam shape are found. Furthermore, a decrease in spatter formation during deep penetration welding is detected for the elliptical beam profile, which correlates with an oscillation movement of the capillary.
CRIS-Team Scopus Importer:2024-02-16
Keywords: Beam shaping; Copper; Diffractive optical element; Green laser radiation; Laser beam welding; Process observation

The industry transformation from fossil fuels to hydrogen will reorganize value chains: Big picture and case studies for Germany - Egerer, Jonas; Farhang-Damghani, Nima; Grimm, Veronika; Runge, Philipp (Applied Energy, 2024)
In many industries, low-carbon hydrogen will substitute fossil fuels in the course of the transformation to climate neutrality. This paper contributes to understanding this transformation. This paper provides an overview of energy- and emission-intensive industry sectors with great potential to defossilize their production processes with hydrogen. An assessment of future hydrogen demand for various defossilization strategies in Germany that rely on hydrogen as a feedstock or as an energy carrier to a different extent in the sectors steel, chemicals, cement, lime, glass, as well as pulp and paper is carried out. Results indicate that aggregate industrial hydrogen demand in those industries would range between 197 TWh and 298 TWh if production did not relocate abroad for any industry sector. The range for hydrogen demand is mainly due to differences in the extent of hydrogen utilization as compared to alternative transformation paths for example based on electrification. The attractiveness of production abroad is then assessed based on the prospective comparative cost advantage of relocating parts of the value chain to excellent production sites for low-carbon hydrogen. Case studies are provided for the steel industry, as well as the chemical industry with ethylene production through methanol and the production of urea on the basis of ammonia. The energy cost of the respective value chains in Germany is then compared to the case of value chains partly located in regions with excellent conditions for renewable energies and hydrogen production. The results illustrate, that at least for some processes – as ammonia production – relocation to those favorable regions may occur due to substantial comparative cost advantages.
CRIS-Team Scopus Importer:2024-02-02
Keywords: Ammonia and urea; Hydrogen-based processes; Industrial hydrogen demand scenarios; Iron and steel; Methanol and ethylene; Transformation of German industry; Value chains

The non-stop disjoint trajectories problem - Hoch, Benno; Liers-Bergmann, Frauke; Neumann, Sarah; Zaragoza Martínez, Francisco Javier (Discrete Optimization, 2024)
Consider an undirected network with traversal times on its edges and a set of commodities with connection requests from sources to destinations and release dates. The non-stop disjoint trajectories problem is to find trajectories that fulfill all requests, such that the commodities never meet. In this extension to the NP-complete disjoint paths problem, trajectories must satisfy a non-stop condition, which disallows waiting at vertices or along arcs. This problem variant appears, for example, when disjoint aircraft trajectories shall be determined or in bufferless packet routing. We study the border of tractability for feasibility and optimization problems on three graph classes that are frequently used where space and time are discretized simultaneously: the path, the grid, and the mesh. We show that if all commodities have a common release date, feasibility can be decided in polynomial time on paths. For the unbounded mesh and unit-costs, we show how to construct optimal trajectories. In contrast, if commodities have individual release intervals and turns are forbidden, then even feasibility is NP-complete for the path. For the mesh and arbitrary edge costs, with individual release dates and turning abilities of commodities restricted to at most 90°, we show that optimization and approximation are not fixed-parameter tractable.
CRIS-Team Scopus Importer:2024-04-26
Keywords: Combinatorial optimization; Complexity; Disjoint paths; Trajectory planning

The Production Process Chain of Axial Flux Motors: A Comparative Study - Baader, Marcel; Hahn, Roman; Franke, Jörg; Kampker, Achim; Born, Henrik; Hartmann, Sebastian; Drexler, David; Parspour, Nejila; Schäfer, Adrian; Pecha, Urs (2024 1st International Conference on Production Technologies and Systems for E-Mobility (EPTS), 2024)
As the global transition to electrification and green energy intensifies, the significance of advanced electric motor technology grows. Axial flux motors (AFM) stand out with their unique design and inherent advantages, positioning them as a promising solution in this sustainability-driven shift. This paper presents a comparative analysis of the manufacturing process chain for AFMs, crucial components in diverse sectors such as electric vehicles, renewable energy systems, and industrial automation. AFMs offer distinct advantages over traditional radial flux motors, including high power density, compact size, and efficiency, underscoring the importance of optimizing their production process for enhanced performance and reliability. Beginning with an overview of AFM technology and its wide application potentials and topologies, the paper examines each stage of a potential manufacturing process chain. These stages encompass design and engineering considerations, winding techniques for stator components, fabrication methods for core laminations, and magnet assembly processes. By examining various methods and processes, their impact on impact on automation, manufacturability, production efficiency as well as quality will be assessed, and the crucial role of AFMs in promoting sustainable technological advancements across industries and in shaping a greener future is highlighted.<br
CRIS-Team Scopus Importer:2024-08-02
Keywords: Stator cores; Power system measurements; Manufacturing processes; Automation; Magnetic cores; Windings; Stator windings; electric motor; axial flux motor; manufacturing; electric motor production; process chain

Toward High-Energy-Density Fuels for Direct Liquid Organic Hydrogen Carrier Fuel Cells: Electrooxidation of 1-Cyclohexylethanol - Fusek, Lukáš; Briega-Martos, Valentín; Minichová, Maria; Fromm, Lukas; Franz, Evanie; Yang, Juntao; Görling, Andreas; Mayrhofer, Karl J.J.; Wasserscheid, Peter; Cherevko, Serhiy; Brummel, Olaf; Libuda, Jörg (Journal of Physical Chemistry Letters, 2024)
Electrochemically active liquid organic hydrogen carriers (EC-LOHCs) can be used directly in fuel cells; so far, however, they have rather low hydrogen storage capacities. In this work, we study the electrooxidation of a potential EC-LOHC with increased energy density, 1-cyclohexylethanol, which consists of two storage functionalities (a secondary alcohol and a cyclohexyl group). We investigated the product spectrum on low-index Pt single-crystal surfaces in an acidic environment by combining cyclic voltammetry, chronoamperometry, and in situ infrared spectroscopy, supported by density functional theory. We show that the electrooxidation of 1-cyclohexylethanol is a highly structure-sensitive reaction with activities Pt(111) ≫ Pt(100) > Pt(110). Most importantly, we demonstrate that 1-cyclohexylethanol can be directly converted to acetophenone, which desorbs from the electrode surface. However, decomposition products are formed, which lead to poisoning. If the latter side reactions could be suppressed, the electrooxidation of 1-cyclohexylethanol would enable the development of EC-LOHCs with greatly increased hydrogen storage capacities.
CRIS-Team Scopus Importer:2024-03-15

Towards a circular economy for electrical products: A systematic literature review and research agenda for automated recycling - Bründl, Patrick; Scheck, Albert; Nguyen, Huong Giang; Franke, Jörg (Robotics and Computer-Integrated Manufacturing, 2024)
The growing need for sustainable waste management, especially for electronic waste (e-waste), has gained attention due to its environmental and economic implications. Despite the technological evolution of electronic products, their short lifespan and frequent replacement have led to a surge in discarded equipment. In particular, e-waste is rich in valuable materials, often exceeding natural ores. The challenge lies in the recycling of complex products, such as electrical or mechatronic systems, which consist of different materials. In addition, many of these discarded products are shipped to low-cost countries, leading to environmental concerns and health risks for workers. This paper explores the potential of automating the recycling process to address these challenges. This systematic literature review examines the emerging field of automated recycling for electronic products, with a focus on the transition to a circular economy. The results show that the literature on automated disassembly is highly focused on intelligent scheduling strategies, object recognition, sorting, tooling, disassembly, and robotic solutions, with particular emphasis on robotic cell design, robot kinematics, and human-robot collaboration. This review makes a significant contribution to the growing academic discourse on automated recycling and the circular economy, while laying the groundwork for future research and innovation by postulating a comprehensive research agenda.
CRIS-Team Scopus Importer:2023-12-15
Keywords: Automation; Control cabinet; Disassembly; Electrical products; Mechatronic products; Recycling; Systematic literature review

Toward Self-Driven Autonomous Material and Device Acceleration Platforms (AMADAP) for Emerging Photovoltaics Technologies - Zhang, Jiyun; Hauch, Jens A.; Brabec, Christoph J. (Accounts of Chemical Research, 2024)
Conspectus In the ever-increasing renewable-energy demand scenario, developing new photovoltaic technologies is important, even in the presence of established terawatt-scale silicon technology. Emerging photovoltaic technologies play a crucial role in diversifying material flows while expanding the photovoltaic product portfolio, thus enhancing security and competitiveness within the solar industry. They also serve as a valuable backup for silicon photovoltaic, providing resilience to the overall energy infrastructure. However, the development of functional solar materials poses intricate multiobjective optimization challenges in a large multidimensional composition and parameter space, in some cases with millions of potential candidates to be explored. Solving it necessitates reproducible, user-independent laboratory work and intelligent preselection of innovative experimental methods. Materials acceleration platforms (MAPs) seamlessly integrate robotic materials synthesis and characterization with AI-driven data analysis and experimental design, positioning them as enabling technologies for the discovery and exploration of new materials. They are proposed to revolutionize materials development away from the Edisonian trial-and-error approaches to ultrashort cycles of experiments with exceptional precision, generating a reliable and highly qualitative data situation that allows training machine learning algorithms with predictive power. MAPs are designed to assist the researcher in multidimensional aspects of materials discovery, such as material synthesis, precursor preparation, sample processing and characterization, and data analysis, and are drawing escalating attention in the field of energy materials. Device acceleration platforms (DAPs), however, are designed to optimize functional films and layer stacks. Unlike MAPs, which focus on material discovery, a central aspect of DAPs is the identification and refinement of ideal processing conditions for a predetermined set of materials. Such platforms prove especially invaluable when dealing with “disordered semiconductors,” which depend heavily on the processing parameters that ultimately define the functional properties and functionality of thin film layers. By facilitating the fine-tuning of processing conditions, DAPs contribute significantly to the advancement and optimization of disordered semiconductor devices, such as emerging photovoltaics. In this Account, we review the recent advancements made by our group in automated and autonomous laboratories for advanced material discovery and device optimization with a strong focus on emerging photovoltaics, such as solution-processing perovskite solar cells and organic photovoltaics. We first introduce two MAPs and two DAPs developed in-house: a microwave-assisted high-throughput synthesis platform for the discovery of organic interface materials, a multipurpose robot-based pipetting platform for the synthesis of new semiconductors and the characterization of thin film semiconductor composites, the SPINBOT system, which is a spin-coating DAP with the potential to optimize complex device architectures, and finally, AMANDA, a fully integrated and autonomously operating DAP. Notably, we underscore the utilization of a robot-based high-throughput experimentation technique to address the common optimization challenges encountered in extensive multidimensional composition and parameter spaces pertaining to organic and perovskite photovoltaics materials. Finally, we briefly propose a holistic concept and technology, a self-driven autonomous material and device acceleration platform (AMADAP) laboratory, for autonomous functional solar materials discovery and development. We hope to discover how AMADAP can be further strengthened and universalized with advancing development of hardware and software infrastructures in the future.
CRIS-Team Scopus Importer:2024-05-10